Method of alkali metal-selenium secondary battery containing a graphene-based separator layer

ABSTRACT

One embodiment of the invention is method of inhibiting the shuttle effect by preventing migration of selenium or metal selenide ions from a cathode to an anode of an alkali metal-selenium battery, the method comprising: (a) combining an anode active material layer, a cathode active material layer, an electrically insulating porous separator disposed between the anode active material layer and the cathode active material layer, and electrolyte to form an alkali metal-selenium battery cell, and (b) implementing a porous trapping layer, having a thickness from 5 nm to 100 μm, between the cathode active material layer and the electrically insulating porous separator to trap selenium or metal selenide ions that are dissolved in the electrolyte from the cathode active material layer. Such a method enables the formation of an alkali metal-selenium battery exhibiting a long cycle life.

FIELD OF THE INVENTION

The present invention is related to a unique separator structure in asecondary or rechargeable alkali metal-selenium battery, including thelithium-selenium battery, sodium-selenium battery, andpotassium-selenium battery, and a method of producing same.

BACKGROUND

Rechargeable lithium-ion (Li-ion) and lithium metal batteries (includingLi-sulfur and Li metal-air batteries) are considered promising powersources for electric vehicle (EV), hybrid electric vehicle (HEV), andportable electronic devices, such as lap-top computers and mobilephones. Lithium as a metal element has the highest capacity (3,861mAh/g) compared to any other metal or metal-intercalated compound as ananode active material (except Li_(4.4)Si, which has a specific capacityof 4,200 mAh/g). Hence, in general, Li metal batteries have asignificantly higher energy density than lithium ion batteries.

Historically, rechargeable lithium metal batteries were produced usingnon-lithiated compounds having relatively high specific capacities, suchas TiS₂, MoS₂, MnO₂, CoO₂, and V₂O₅, as the cathode active materials,which were coupled with a lithium metal anode. When the battery wasdischarged, lithium ions were transferred from the lithium metal anodethrough the electrolyte to the cathode, and the cathode becamelithiated. Unfortunately, upon repeated charges/discharges, the lithiummetal resulted in the formation of dendrites at the anode thatultimately grew to penetrate through the separator, causing internalshorting and explosion. As a result of a series of accidents associatedwith this problem, the production of these types of secondary batterieswas stopped in the early 1990's, giving ways to lithium-ion batteries.

In lithium-ion batteries, pure lithium metal sheet or film was replacedby carbonaceous materials as the anode. The carbonaceous materialabsorbs lithium (through intercalation of lithium ions or atoms betweengraphene planes, for instance) and desorbs lithium ions during there-charge and discharge phases, respectively, of the lithium ion batteryoperation. The carbonaceous material may comprise primarily graphitethat can be intercalated with lithium and the resulting graphiteintercalation compound may be expressed as Li_(x)C₆, where x istypically less than 1.

Although lithium-ion (Li-ion) batteries are promising energy storagedevices for electric drive vehicles, state-of-the-art Li-ion batterieshave yet to meet the cost and performance targets. Li-ion cellstypically use a lithium transition-metal oxide or phosphate as apositive electrode (cathode) that de/re-intercalates Li⁺ at a highpotential with respect to the carbon negative electrode (anode). Thespecific capacity of lithium transition-metal oxide or phosphate basedcathode active material is typically in the range of 140-180 mAh/g. As aresult, the specific energy of commercially available Li-ion cells istypically in the range of 120-240 Wh/kg, most. These specific energyvalues are two to three times lower than what would be required forbattery-powered electric vehicles to be widely accepted.

With the rapid development of hybrid (HEV), plug-in hybrid electricvehicles (HEV), and all-battery electric vehicles (EV), there is anurgent need for anode and cathode materials that provide a rechargeablebattery with a significantly higher specific energy, higher energydensity, higher rate capability, long cycle life, and safety. Two of themost promising energy storage devices are the lithium-sulfur (Li—S) celland lithium-selenium (Li—Se) cell since the theoretical capacity of Liis 3,861 mAh/g, that of S is 1,675 mAh/g, and that of Se is 675 mAh/g.Compared with conventional intercalation-based Li-ion batteries, Li—Sand Li—Se cells have the opportunity to provide a significantly higherenergy density (a product of capacity and voltage). With a significantlyhigher electronic conductivity, Se is a more effective cathode activematerial and, as such, Li—Se potentially can exhibit a higher ratecapability.

However, Li—Se cell is plagued with several major technical problemsthat have hindered its widespread commercialization:

-   (1) All prior art Li—Se cells have dendrite formation and related    internal shorting issues;-   (2) The cell tends to exhibit significant capacity decay during    discharge-charge cycling. This is mainly due to the high solubility    of selenium and lithium poly selenide anions formed as reaction    intermediates during both discharge and charge processes in the    polar organic solvents used in electrolytes. During cycling, the    anions can migrate through the separator to the Li negative    electrode whereupon they are reduced to solid precipitates, causing    active mass loss. In addition, the solid product that precipitates    on the surface of the positive electrode during discharge becomes    electrochemically irreversible, which also contributes to active    mass loss. This phenomenon is commonly referred to as the Shuttle    Effect. This process leads to several problems: high self-discharge    rates, loss of cathode capacity, corrosion of current collectors and    electrical leads leading to loss of electrical contact to active    cell components, fouling of the anode surface giving rise to    malfunction of the anode, and clogging of the pores in the cell    membrane separator which leads to loss of ion transport and large    increases in internal resistance in the cell.-   (3) Presumably, nanostructured mesoporous carbon materials could be    used to hold the Se or lithium polyselenide in their pores,    preventing large out-flux of these species from the porous carbon    structure through the electrolyte into the anode. However, the    fabrication of the proposed highly ordered mesoporous carbon    structure requires a tedious and expensive template-assisted    process. It is also challenging to load a large proportion of    selenium into the mesoscaled pores of these materials using a    physical vapor deposition or solution precipitation process.    Typically the maximum loading of Se in these porous carbon    structures is less than 50% by weight (i.e. the amount of active    material is less than 50%; more than 50% being inactive materials).

Despite the various approaches proposed for the fabrication of highenergy density Li—Se cells, there remains a need for cathode materials,production processes, and cell operation methods that retard theout-diffusion of Se or lithium polyselenide from the cathodecompartments into other components in these cells, improve theutilization of electro-active cathode materials (Se utilizationefficiency), and provide rechargeable Li—Se cells with high capacitiesover a large number of cycles.

Most significantly, lithium metal (including pure lithium, lithiumalloys of high lithium content with other metal elements, orlithium-containing compounds with a high lithium content; e.g. >80% orpreferably >90% by weight Li) still provides the highest anode specificcapacity as compared to essentially all other anode active materials(except pure silicon, but silicon has pulverization issues). Lithiummetal would be an ideal anode material in a lithium-selenium secondarybattery if dendrite related issues could be addressed.

Sodium metal (Na) and potassium metal (K) have similar chemicalcharacteristics to Li and the selenium cathode in sodium-selenium cells(Na—Se batteries) or potassium-selenium cells (K—Se) face the sameissues observed in Li—S batteries, such as: (i) low active materialutilization rate, (ii) poor cycle life, and (iii) low Coulumbicefficiency. Again, these drawbacks arise mainly from insulating natureof Se, dissolution of polyselenide intermediates in liquid electrolytes(and related Shuttle effect), and large volume change duringcharge/discharge.

Hence, an object of the present invention is to provide a rechargeableLi—Se battery that exhibits an exceptionally high specific energy orhigh energy density. One particular technical goal of the presentinvention is to provide a Li metal-selenium or Li ion-selenium cell witha cell specific energy greater than 300 Wh/Kg, preferably greater than350 Wh/Kg, and more preferably greater than 400 Wh/Kg (all based on thetotal cell weight).

It may be noted that in most of the open literature reports (scientificpapers) and patent documents, scientists or inventors choose to expressthe cathode specific capacity based on the selenium or lithiumpolyselenide weight alone (not the total cathode composite weight), butunfortunately a large proportion of non-active materials (those notcapable of storing lithium, such as conductive additive and binder) istypically used in their Li—Se cells. For practical use purposes, it ismore meaningful to use the cathode composite weight-based capacityvalue.

A specific object of the present invention is to provide a rechargeablelithium-selenium cell based on rational materials and battery designsthat overcome or significantly reduce the following issues commonlyassociated with conventional Li—Se cells: (a) dendrite formation(internal shorting); (b) low electric and ionic conductivities ofselenium, requiring large proportion (typically 30-55%) of non-activeconductive fillers and having significant proportion of non-accessibleor non-reachable selenium or lithium polyselenide); (c) dissolution oflithium polyselenide in electrolyte and migration of dissolved lithiumpolyselenide from the cathode to the anode (which irreversibly reactwith lithium at the anode), resulting in active material loss andcapacity decay (the shuttle effect); and (d) short cycle life.

In addition to overcoming the aforementioned problems, another object ofthe present invention is to provide a simple, cost-effective, andeasy-to-implement approach to preventing potential Li metaldendrite-induced internal short circuit and thermal runaway problems inLi metal-selenide batteries.

SUMMARY OF THE INVENTION

The present invention provides an alkali metal-selenium batterycomprising: (A) an anode containing an anode active material layer andan optional anode current collector supporting this anode activematerial layer; (B) a cathode containing a cathode active material layerand an optional cathode current collector supporting this cathode activematerial layer, wherein the cathode active material layer contains aselenium-containing material, as a cathode active material, selectedfrom selenium, a selenium-carbon hybrid, a selenium-graphite hybrid, aselenium-graphene hybrid, a conducting polymer-selenium hybrid, a metalselenide, a Se alloy or mixture with Sn, Sb, Bi, S, or Te, a seleniumcompound, or a combination thereof; (C) an electrolyte in ionic contactwith the cathode and the anode and an optional porous separator that iselectronically insulating and separates the anode and the cathode; and(D) a graphene separator layer containing a solid graphene foam, paperor fabric that is permeable to lithium ions or sodium ions but issubstantially non-permeable to selenium or metal selenide, wherein thegraphene separator layer is disposed between the anode active materiallayer and the cathode active material layer and is in physical contactwith the cathode active material layer but not in physical contact withthe anode active material layer and wherein the graphene separator layercontains pristine graphene sheets having less than 0.01% by weight ofnon-carbon elements or non-pristine graphene sheets having 0.01% to 20%by weight of non-carbon elements, wherein said non-pristine graphene isselected from graphene oxide, reduced graphene oxide, graphene fluoride,graphene chloride, graphene bromide, graphene iodide, hydrogenatedgraphene, nitrogenated graphene, boron-doped graphene, nitrogen-dopedgraphene, chemically functionalized graphene, or a combination thereof.

Typically, this graphene separator layer is electronically conductingand, hence, cannot be in physical contact with both the anode activematerial layer and the cathode active material layer. As such, thisgraphene separator layer is herein disposed to be in physical contactwith the cathode layer, but not the anode layer. The electronicallyinsulating porous separator layer (e.g. the porouspolyethylene-polypropylene copolymer membrane commonly used in thelithium-ion battery industry) is not required if the electrolyte in thealkali metal-selenium cell is a solid polymer electrolyte or solid-stateelectrolyte. If this electronically insulating porous separator layer ispresent, the graphene separator layer is disposed between the insulatingporous separator layer and the cathode active material layer.

The graphene separator layer is a discrete layer separate from(independent of) the cathode active material layer. The cathode activematerial layer itself can contain graphene sheets as a conductiveadditive or as an encapsulating material that embraces or encapsulatesparticles of a selenium-containing material. Even in such a situation,there is an additional, separate graphene separator layer implementedbetween this cathode active material layer and the electronicallyinsulating porous separator layer.

The solid graphene foam may contain a three-dimensional network ofinterconnected and ordered open cells. The solid graphene foam, whenmeasured without the metal, has a density ranging from about 0.001 g/cm³to about 1.7 g/cm³, more preferably and typically from about 0.01 g/cm³to about 1.5 g/cm³, and most preferably from about 0.01 g/cm³ to about1.2 g/cm³.

Preferably, the graphene separator layer has a thickness from 5 nm to100 μm, more preferably from 10 nm to 20 μm. The solid graphene foam,paper, or fabric layer preferably contains pores having a size from 0.5nm to 50 nm.

The solid separator layer has a graphene-based composition and structurethat is capable of blocking selenium or metal selenide species dissolvedin the electrolyte in the cathode side from migrating to the anode side,thereby reducing or eliminating the shuttle effect. In certainembodiments, this graphene separator layer traps those dissolved speciesand retains them in the cathode side. These trapped or blocked speciesremain capable of reacting with or storing lithium ions in the cathodeside.

In certain embodiments, the solid graphene foam, paper or fabric in thegraphene separator layer can optionally further contain a carbon orgraphite filler selected from a carbon or graphite fiber, carbon orgraphite nanofiber, carbon nanotube, carbon nanorod, mesophase carbonparticle, mesocarbon microbead, expanded graphite flake, needle coke,carbon black or acetylene black, activated carbon, or a combinationthereof.

The chemically functionalized graphene sheets may have a chemicalfunctional group selected from alkyl or aryl silane, alkyl or aralkylgroup, hydroxyl group, carboxyl group, carboxylic group, amine group,sulfonate group (—SO₃H), aldehydic group, quinoidal, fluorocarbon, or acombination thereof.

In certain embodiments, the chemically functionalized graphene comprisesgraphene sheets having a chemical functional group selected from aderivative of an azide compound selected from the group consisting of2-azidoethanol, 3-azidopropan-1-amine, 4-(2-azidoethoxy)-4-oxobutanoicacid, 2-azidoethyl-2-bromo-2-methylpropanoate, chlorocarbonate,azidocarbonate, dichlorocarbene, carbene, aryne, nitrene,(R-)-oxycarbonyl nitrenes, where R=any one of the following groups,

and combinations thereof.

In certain embodiments, the chemically functionalized graphene comprisesgraphene sheets having a chemical functional group selected from anoxygenated group selected from the group consisting of hydroxyl,peroxide, ether, keto, and aldehyde.

In some preferred embodiments, the chemically functionalized graphenecomprises graphene sheets having a chemical functional group selectedfrom the group consisting of —SO₃H, —COOH, —NH₂, —OH, —R′CHOH, —CHO,—CN, —COCl, halide, —COSH, —SH, —COOR′, —SR′, —SiR′₃, —Si(—O—SiR′₂—)OR′,—R″, Li, AlR′₂, Hg—X, TlZ₂ and Mg—X; wherein y is an integer equal to orless than 3, R′ is hydrogen, alkyl, aryl, cycloalkyl, or aralkyl,cycloaryl, or poly(alkylether), R″ is fluoroalkyl, fluoroaryl,fluorocycloalkyl, fluoroaralkyl or cycloaryl, X is halide, and Z iscarboxylate or trifluoroacetate, and combinations thereof.

In some embodiments, the chemically functionalized graphene comprisesgraphene sheets having a chemical functional group selected from thegroup consisting of amidoamines, polyamides, aliphatic amines, modifiedaliphatic amines, cycloaliphatic amines, aromatic amines, anhydrides,ketimines, diethylenetriamine (DETA), triethylene-tetramine (TETA),tetraethylene-pentamine (TEPA), polyethylene polyamine, polyamine epoxyadduct, phenolic hardener, non-brominated curing agent, non-aminecuratives, and combinations thereof.

The chemically functionalized graphene may comprise graphene sheetshaving a chemical functional group selected from OY, NHY, O═C—OY,P═C—NR′Y, O═C—SY, O═C—Y, —CR′1-OY, N′Y or C′Y, and Y is a functionalgroup of a protein, a peptide, an amino acid, an enzyme, an antibody, anucleotide, an oligonucleotide, an antigen, or an enzyme substrate,enzyme inhibitor or the transition state analog of an enzyme substrateor is selected from R′—OH, R′—NR′₂, R′SH, R′CHO, R′CN, R′X, R′N⁺(R′)₃X⁻,R′SiR′₃, R′Si(—OR′—)_(y)R′_(3−y), R′Si(—O—SiR′₂—)OR′, R′—R″, R′—N—CO,(C₂H₄O—)_(w)H, (—C₃H₆O—)_(w)H, (—C₂H₄O)_(w)—R′, (C₃H₆O)_(w)—R′, R′, andw is an integer greater than one and less than 200.

In certain embodiments, the invented alkali metal-selenium batteryfurther comprises an anode current collector and/or an additionalseparate cathode current collector. The alkali metal-selenium batterymay be selected from a rechargeable lithium-selenium cell,sodium-selenium cell, potassium-selenium cell, lithium ion-seleniumcell, sodium ion-selenium cell, or potassium ion-selenium cell.

In the invented alkali metal-selenium battery, the electrolyte may beselected from polymer electrolyte, polymer gel electrolyte, compositeelectrolyte, ionic liquid electrolyte, non-aqueous liquid electrolyte,soft matter phase electrolyte, solid-state electrolyte, or a combinationthereof.

The electrolyte may contain an alkali salt selected from lithiumperchlorate (LiClO₄), lithium hexafluorophosphate (LiPF₆), lithiumborofluoride (LiBF₄), lithium hexafluoroarsenide (LiAsF₆), lithiumtrifluoro-metasulfonate (LiCF₃SO₃), bis-trifluoromethyl sulfonylimidelithium (LiN(CF₃SO₂)₂, lithium bis(oxalato)borate (LiBOB), lithiumoxalyldifluoroborate (LiBF₂C₂O₄), lithium oxalyldifluoroborate(LiBF₂C₂O₄), lithium nitrate (LiNO₃), Li-fluoroalkyl-fhosphates(LiPF3(CF₂CF₃)₃), lithium bisperfluoroethysulfonylimide (LiBETI), anionic liquid salt, sodium perchlorate (NaClO₄), potassium perchlorate(KC10₄), sodium hexafluorophosphate (NaPF₆), potassiumhexafluorophosphate (KPF₆), sodium borofluoride (NaBF₄), potassiumborofluoride (KBF₄), sodium hexafluoroarsenide, potassiumhexafluoroarsenide, sodium trifluoro-metasulfonate (NaCF₃SO₃), potassiumtrifluoro-metasulfonate (KCF₃SO₃), bis-trifluoromethyl sulfonylimidesodium (NaN(CF₃SO₂)₂), sodium trifluoromethanesulfonimide (NaTFSI),bis-trifluoromethyl sulfonylimide potassium (KN(CF₃SO₂)₂), or acombination thereof.

In the alkali metal-selenium battery, the electrolyte can contain asolvent selected from ethylene carbonate (EC), dimethyl carbonate (DMC),methylethyl carbonate (MEC), diethyl carbonate (DEC), ethyl propionate,methyl propionate, propylene carbonate (PC), gamma-butyrolactone (γ-BL),acetonitrile (AN), ethyl acetate (EA), propyl formate (PF), methylformate (MF), toluene, xylene or methyl acetate (MA), fluoroethylenecarbonate (FEC), vinylene carbonate (VC), allyl ethyl carbonate (AEC),1,3-dioxolane (DOL), 1,2-dimethoxyethane (DME), tetraethylene glycoldimethylether (TEGDME), Poly(ethylene glycol) dimethyl ether (PEGDME),diethylene glycol dibutyl ether (DEGDBE), 2-ethoxyethyl ether (EEE),sulfone, sulfolane, room temperature ionic liquid, or a combinationthereof.

In the invented alkali metal-selenium battery of claim, the anode activematerial may be selected from Li, Na, K, an alloy thereof, a compoundthereof, graphite, carbon, Si, SiO, Sn, SnO₂, a transition metal oxide,or a combination thereof.

The cathode active material may be selected from Se and/or metalselenide. The selenium or metal selenide is preferably in the form ofthin coating or particles preferably having a thickness of diameter from0.5 nm to 100 nm (more preferably from 1 nm to 10 nm). The cathodeactive material may further comprise a second element selected from Sn,Sb, Bi, S, Te, or a combination thereof and the weight of the secondelement is less than the weight of selenium. The second element may bemixed with selenium (Se) to form a mixture or alloy. The second element,the mixture, or the alloy may be preferably in a nanoparticle ornanocoating form having a diameter or thickness from 0.5 nm to 100 nm.Se, metal selenide, and/or the second element preferably resides in thepores or bonded to pore walls of a carbon-based, graphite-based, orgraphene-based foam.

The invention also includes a process for producing the grapheneseparator layer. The graphene paper may be produced from discretegraphene sheets using any known paper-making procedure. The graphenefabric may be made by making graphene sheets into a woven or non-wovenstructure. These procedures are well-known in the art. However, one mustpreferably make the graphene-based paper or fabric to contain poreshaving a pore size in the range of 0.5 nm to 50 nm, preferably from 1 nmto 10 nm.

In certain embodiments, the graphene separator layer contains a layer ofgraphene foam and the process comprises: (a) preparing a graphenedispersion having multiple graphene sheets dispersed in a liquid medium,wherein the graphene sheets are selected from a pristine graphene or anon-pristine graphene material, having a content of non-carbon elementsgreater than 2% by weight, selected from graphene oxide, reducedgraphene oxide, graphene fluoride, graphene chloride, graphene bromide,graphene iodide, hydrogenated graphene, nitrogenated graphene,chemically functionalized graphene, or a combination thereof and whereinsaid graphene dispersion contains an optional blowing agent having ablowing agent-to-graphene material weight ratio from 0/1.0 to 1.0/1.0;(b) dispensing and depositing the graphene dispersion onto a surface ofa supporting substrate to form a wet layer of graphene; (c) partially orcompletely removing the liquid medium from the wet layer of graphene toform a dried layer of graphene; and (d) heat treating the dried layer ofgraphene at a first heat treatment temperature selected from 80° C. to3,200° C. at a desired heating rate sufficient to induce volatile gasmolecules from the non-carbon elements or to activate the blowing agentfor producing a sheet or roll of solid graphene foam having multiplepores (cells) and pore walls (cell walls) containing graphene sheets.The dispensing and depositing procedure may include subjecting thegraphene dispersion to an orientation-inducing stress.

In certain embodiments, the process further includes a step ofheat-treating the solid graphene foam at a second heat treatmenttemperature higher than the first heat treatment temperature for alength of time sufficient for increasing the thermal conductivity andelectrical conductivity of the solid graphene foam wherein the porewalls contain stacked graphene planes having an inter-plane spacing d₀₀₂from 0.3354 nm to 0.36 nm and a content of non-carbon elements less than2% by weight.

In certain embodiments, the graphene sheets contain pristine grapheneand said graphene dispersion contains a blowing agent having a blowingagent-to-pristine graphene weight ratio from 0.01/1.0 to 1.0/1.0.

The blowing agent is a physical blowing agent, a chemical blowing agent,a mixture thereof, a dissolution-and-leaching agent, or a mechanicallyintroduced blowing agent.

The process may be a roll-to-roll process wherein said steps (b) and (c)include feeding said supporting substrate from a feeder roller to adeposition zone, continuously or intermittently depositing the graphenedispersion onto a surface of the supporting substrate to form the wetlayer of graphene thereon, drying the wet layer of graphene, andcollecting the dried layer of graphene material deposited on thesupporting substrate on a collector roller

The first heat treatment temperature is preferably selected from 100° C.to 1,500° C. The second heat treatment temperature may include at leasta temperature selected from (A) 300-1,500° C., (B) 1,500-2,100° C., or(C) 2,100-3,200° C.

The step (d) of heat treating the dried layer of graphene at a firstheat treatment temperature may be conducted under a compressive stress.The process may further comprise a compression step to reduce athickness, a pore size, or a porosity level of the solid graphene foam.

In certain preferred embodiments, the process may further comprise astep of chemically functionalizing graphene sheets in the solid graphenefoam, after step (d), to promote or facilitate entrapment of dissolvedselenium or metal selenide species. The chemical functionalization stepmay include attaching a functional group recited earlier in thissection.

Prior to the step of chemically functionalizing graphene sheets, thesegraphene sheets may be essentially free of any significant amount ofoxygen and hydrogen and they are no longer graphene oxide.

The graphene dispersion may further contain particles or fibrils of ametal, glass, ceramic, carbon or graphite filler to induce orientationof the graphene sheets inclined at an angle of 15-90 degrees relative tosaid paper sheet plane. The carbon or graphite filler is selected from acarbon or graphite fiber, carbon or graphite nanofiber, carbon nanotube,carbon nanorod, mesophase carbon particle, mesocarbon microbead,expanded graphite flake, needle coke, carbon black or acetylene black,activated carbon, or a combination thereof. The filler-to-graphene ratiois from 1/100 to 1/1.

In certain embodiments, the graphene sheets in the graphene dispersionoccupy a weight fraction of 0.1% to 25% (preferably from 3% to 15%)based on the total weight of graphene sheets and liquid medium combined.

In certain embodiments, the graphene dispersion has greater than 3% byweight of graphene or graphene oxide sheets dispersed in the fluidmedium to form a liquid crystal phase, which promotes alignment ofgraphene sheets during the sheet forming procedure.

In this process, the solid graphene foam typically has a density rangingfrom about 0.01 g/cm³ to about 1.7 g/cm³. In this process, the graphenedispersion may further contain a carbon or graphite filler selected froma carbon or graphite fiber, carbon or graphite nanofiber, carbonnanotube, carbon nanorod, mesophase carbon particle, mesocarbonmicrobead, expanded graphite flake, needle coke, carbon black oracetylene black, activated carbon, or a combination thereof and thecarbon or graphite filler is incorporated into the pore walls.

The process may be a roll-to-roll process wherein said steps (b) and (c)include feeding said supporting substrate from a feeder roller to adeposition zone, continuously depositing the graphene dispersion onto asurface of the supporting substrate to form the wet layer of graphenemixture thereon, drying the wet layer of graphene, and collecting thedried layer of graphene mixture deposited on the supporting substrate ona collector roller.

In certain embodiments, step (d) of heat treating the dried layer ofgraphene mixture at a first heat treatment temperature is conductedunder a compressive stress.

The solid graphene separator layer can contain a sheet (layer) ofgraphene paper or graphene-based fabric. Again, the production ofgraphene paper is well known in the art. The production of fabric isalso well-known in the art.

The solid graphene separator layer can have a thickness from 5 nm to 100μm, preferably or more typically from 10 nm to 50 and most preferablyfrom 100 nm to 20 μm.

The process may further include a step of combining an anode, a seleniumcathode layer, an electrolyte and an optional electrically insulatingporous separator layer, and the invented solid graphene separator layer,together to form an alkali metal-selenium battery cell.

The present invention also provides a method of inhibiting the shuttleeffect by preventing migration of selenium or metal selenide ions from acathode to an anode of an alkali metal-selenium battery, the methodcomprising: (a) combining an anode active material layer, a cathodeactive material layer, an electrically insulating porous separatordisposed between the anode active material layer and the cathode activematerial layer, and an electrolyte to form an alkali metal-seleniumbattery cell, and (b) implementing a porous trapping layer, having athickness from 5 nm to 100 μm, between the cathode active material layerand the electrically insulating porous separator to trap selenium ormetal selenide ions that are dissolved in the electrolyte from thecathode active material layer.

In the method, the anode active material layer preferably comprises ananode active material, selected from Li, Na, K, an alloy thereof, acompound thereof, graphite, carbon, Si, SiO, Sn, SnO₂, a transitionmetal oxide, or a combination thereof, and an optional anode currentcollector supporting said anode active material layer.

In the method, the cathode active material layer preferably contains aselenium-containing material, as a cathode active material, selectedfrom selenium, a selenium-carbon hybrid, a selenium-graphite hybrid, aselenium-graphene hybrid, a conducting polymer-selenium hybrid, a metalselenide, a Se alloy or mixture with Sn, Sb, Bi, S, or Te, a seleniumcompound, or a combination thereof and an optional cathode currentcollector supporting said cathode active material layer.

In the method, the porous trapping layer preferably comprises a grapheneseparator layer containing a solid graphene foam, paper or fabric thatis permeable to lithium ions or sodium ions but is substantiallynon-permeable to selenium or metal selenide ions, wherein the grapheneseparator layer is disposed between the anode active material layer andthe cathode active material layer and is in physical contact with thecathode active material layer but not in physical contact with the anodeactive material layer and wherein the graphene separator layer containspristine graphene sheets having less than 0.01% by weight of non-carbonelements or non-pristine graphene sheets having 0.01% to 20% by weightof non-carbon elements, wherein the non-pristine graphene is selectedfrom graphene oxide, reduced graphene oxide, graphene fluoride, graphenechloride, graphene bromide, graphene iodide, hydrogenated graphene,nitrogenated graphene, boron-doped graphene, nitrogen-doped graphene,chemically functionalized graphene, or a combination thereof.Preferably, the solid graphene foam, paper or fabric has a densityranging from about 0.001 g/cm³ to about 1.7 g/cm³. Preferably, the solidgraphene foam, paper, or fabric contains pores having a size from 0.5 nmto 50 nm.

In certain embodiments, the porous trapping layer comprises a foam,paper or fabric structure of a carbon or graphite material selected froma carbon or graphite fiber, carbon or graphite nanofiber, carbonnanotube, carbon nanorod, mesophase carbon particle, mesocarbonmicrobead, expanded graphite flake, needle coke, carbon black, acetyleneblack, activated carbon, a combination thereof, or a combination thereofwith graphene sheets.

In the method, for certain embodiments, the carbon or graphite materialis chemically functionalized to have a chemical functional groupattached thereto to promote trapping of selenium or metal selenide ions.

For instance, in certain embodiments, the chemical functional groupattached to the carbon or graphite material is selected from alkyl oraryl silane, alkyl or aralkyl group, hydroxyl group, carboxyl group,carboxylic group, amine group, sulfonate group (—SO₃H), aldehydic group,quinoidal, fluorocarbon, or a combination thereof.

In certain embodiments, the chemical functional group attached to thecarbon or graphite material is selected from a derivative of an azidecompound selected from the group consisting of 2-azidoethanol,3-azidopropan-1-amine, 4-(2-azidoethoxy)-4-oxobutanoic acid,2-azidoethyl-2-bromo-2-methylpropanoate, chlorocarbonate,azidocarbonate, dichlorocarbene, carbene, aryne, nitrene,(R-)-oxycarbonyl nitrenes, where R=any one of the following groups,

and combinations thereof.

In certain embodiments, the chemical functional group attached to thecarbon or graphite material is selected from an oxygenated groupselected from the group consisting of hydroxyl, peroxide, ether, keto,and aldehyde.

In certain embodiments, the chemical functional group attached to thecarbon or graphite material is selected from the group consisting of—SO₃H, —COOH, —NH₂, —OH, —R′CHOH, —CHO, —CN, —COCl, halide, —COSH, —SH,—COOR′, —SR′, —SiR′₃, —Si(—OR′—)_(y)R′_(3−y), —Si(—O—SiR′₂—)OR′, —R″,Li, AlR′₂, Hg—X, TlZ₂ and Mg—X; wherein y is an integer equal to or lessthan 3, R′ is hydrogen, alkyl, aryl, cycloalkyl, or aralkyl, cycloaryl,or poly(alkylether), R″ is fluoroalkyl, fluoroaryl, fluorocycloalkyl,fluoroaralkyl or cycloaryl, X is halide, and Z is carboxylate ortrifluoroacetate, and combinations thereof.

In certain embodiments, the chemical functional group attached to thecarbon or graphite material is selected from the group consisting ofamidoamines, polyamides, aliphatic amines, modified aliphatic amines,cycloaliphatic amines, aromatic amines, anhydrides, ketimines,diethylenetriamine (DETA), triethylene-tetramine (TETA),tetraethylene-pentamine (TEPA), polyethylene polyamine, polyamine epoxyadduct, phenolic hardener, non-brominated curing agent, non-aminecuratives, and combinations thereof.

In certain embodiments, the chemical functional group attached to thecarbon or graphite material is selected from OY, NHY, O═C—OY, P═C—NR′Y,O═C—SY, O═C—Y, —CR′1-OY, N′Y or C′Y, and Y is a functional group of aprotein, a peptide, an amino acid, an enzyme, an antibody, a nucleotide,an oligonucleotide, an antigen, or an enzyme substrate, enzyme inhibitoror the transition state analog of an enzyme substrate or is selectedfrom R′—OH, R′—NR′₂, R′SH, R′CHO, R′CN, R′X, R′N⁺(R′)₃X⁻, R′SiR′₃,R′Si(—O—SiR′₂—)OR′, R′—R″, R′—N—CO, (C₂H₄O—)_(w)H, (—C₃H₆O—)_(w)H,(—C₂H₄O)_(w)—R′, (C₃H₆O)_(w)—R′, R′, and w is an integer greater thanone and less than 200.

The alkali metal-selenium battery may be selected from a rechargeablelithium-selenium cell, sodium-selenium cell, potassium-selenium cell,lithium ion-selenium cell, sodium ion-selenium cell, or potassiumion-selenium cell.

The electrolyte may be selected from polymer electrolyte, polymer gelelectrolyte, composite electrolyte, ionic liquid electrolyte,non-aqueous liquid electrolyte, soft matter phase electrolyte,solid-state electrolyte, or a combination thereof.

The cathode active material layer may contain a cathode active materialSe, metal selenide, a second element selected from Sn, Sb, Bi, S, Te, ora combination thereof and said cathode active material is in a form ofthin coating or particles having a thickness of diameter from 0.5 nm to100 nm. The thin coating or particles preferably reside in pores orbonded to pore walls of a carbon-based, graphite-based, orgraphene-based foam structure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 Schematic drawing illustrating the processes for producingconventional paper, mat, film, and membrane of simply aggregatedgraphite or NGP flakes/platelets. All processes begin with intercalationand/or oxidation treatment of graphitic materials (e.g. natural graphiteparticles).

FIG. 2(A) Schematic of solid graphene foam containing interconnectedpores (open cells).

FIG. 2(B) Schematic of a paper-making procedure for producing graphenepaper.

FIG. 3 A possible mechanism of chemical linking between graphene oxidesheets, which mechanism effectively increases the graphene sheet lateraldimensions.

FIG. 4 In-plane and through-plane electrical conductivity values of someGO-derived graphene foam sheets (prepared by Comma coating, heattreatment, and compression).

FIG. 5(A) Thermal conductivity values vs. specific gravity of the GOsuspension-derived foam produced by the presently invented process,mesophase pitch-derived graphite foam, and Ni foam-template assisted CVDgraphene foam;

FIG. 5(B) Thermal conductivity values of the GO suspension-derived foam,sacrificial plastic bead-templated GO foam, and the hydrothermallyreduced GO graphene foam;

FIG. 5(C) Electrical conductivity data for the GO suspension-derivedfoam produced by the presently invented process and the hydrothermallyreduced GO graphene foam; and

FIG. 6(A) Thermal conductivity values (vs. specific gravity values up to1.02 g/cm³) of the GO suspension-derived foam, mesophase pitch-derivedgraphite foam, and Ni foam-template assisted CVD graphene foam;

FIG. 6(B) Thermal conductivity values of the GO suspension-derived foam,sacrificial plastic bead-templated GO foam, and hydrothermally reducedGO graphene foam (vs. specific gravity values up to 1.02 g/cm³);

FIG. 7 Thermal conductivity values of graphene foam samples derived fromGO and GF (graphene fluoride) as a function of the specific gravity.

FIG. 8 Thermal conductivity values of graphene foam samples derived fromGO and pristine graphene as a function of the final (maximum) heattreatment temperature.

FIG. 9(A) Inter-graphene plane spacing in graphene foam walls asmeasured by X-ray diffraction;

FIG. 9(B) The oxygen content in the GO suspension-derived graphene foam.

FIG. 10 The charge and discharge cycling results of three Li—Se cells,one containing a presently invented separator or ion-trapping layer ofGO-derived graphene foam, one containing a CNF paper-based separator orion-trapping layer, and one containing no separator layer. All threecells contain a cathode active material prepared by ball-milling amixture of Se powder and carbon black powder.

FIG. 11 Ragone plots (cell power density vs. cell energy density) of twoLi metal-selenium cells; one containing a pristine graphene foamseparator and the other not.

FIG. 12 Ragone plots (cell power density vs. cell energy density) of 2alkali metal-selenium cells: a Na—Se cell featuring an open-cell RGOfoam-based separator implemented between the cathode layer andinsulating polymer separator layer and a Na—Se cell without such aconducting separator layer.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

For convenience, the following discussion of preferred embodiments isprimarily based on cathodes for Li—Se cells, but the same or similarmethods are applicable to deposition of Se in the cathode for the Na—Seand K—Se cells. Examples are presented for Li—Se cells, Na—Se cells, andK—Se cells.

The present invention provides an alkali metal-selenium batterycomprising:

(A) an anode containing an anode active material layer and an optionalanode current collector supporting this anode active material layer;(B) a cathode containing a cathode active material layer and an optionalcathode current collector supporting this cathode active material layer,wherein the cathode active material layer contains a selenium-containingmaterial, as a cathode active material, selected from selenium, aselenium-carbon hybrid, a selenium-graphite hybrid, a selenium-graphenehybrid, a conducting polymer-selenium hybrid, a metal selenide, a Sealloy or mixture with Sn, Sb, Bi, S, or Te, a selenium compound, or acombination thereof;(C) an electrolyte in ionic contact with the cathode and the anode andan optional porous separator that is electronically insulating andseparates the anode and the cathode; and(D) a graphene separator layer containing a solid graphene foam, paperor fabric that is permeable to lithium ions or sodium ions but issubstantially non-permeable to selenium or metal selenide, wherein thegraphene separator layer is disposed between the anode active materiallayer and the cathode active material layer and is in physical contactwith the cathode active material layer but not in physical contact withthe anode active material layer.

This discrete, separate graphene separator layer is electronicallyconducting and is independent of and separate from both theelectronically insulating porous layer (the conventional porousseparator) and the cathode active material layer. When/if thisconventional porous separator layer is present, the presently inventedgraphene separator layer is disposed between this conventionalinsulating layer and the cathode active material layer.

The graphene separator layer contains pristine graphene sheets havingless than 0.01% by weight of non-carbon elements or non-pristinegraphene sheets having 0.01% to 20% by weight of non-carbon elements,wherein said non-pristine graphene is selected from graphene oxide,reduced graphene oxide, graphene fluoride, graphene chloride, graphenebromide, graphene iodide, hydrogenated graphene, nitrogenated graphene,boron-doped graphene, nitrogen-doped graphene, chemically functionalizedgraphene, or a combination thereof.

The solid graphene foam may contain be a closed-cell graphene foamstructure. Alternatively, as schematically shown in FIG. 2(A), thegraphene foam may contain an open-cell graphene foam structure havinginterconnected pores (open cells). The solid graphene foam may contain athree-dimensional network of interconnected open cells. The solidgraphene foam typically has a density ranging from about 0.001 g/cm³ toabout 1.7 g/cm³, more preferably and typically from about 0.01 g/cm³ toabout 1.5 g/cm³, and most preferably from about 0.01 g/cm³ to about 0.8g/cm³.

In certain embodiments, the solid graphene foam in the grapheneseparator layer can optionally further contain a carbon or graphitefiller selected from a carbon or graphite fiber, carbon or graphitenanofiber, carbon nanotube, carbon nanorod, mesophase carbon particle,mesocarbon microbead, expanded graphite flake, needle coke, carbon blackor acetylene black, activated carbon, or a combination thereof.

There is no particular restriction of the type of Se cathode that can beused in the presently invented alkali metal-Se battery. For instance,the cathode active material layer may contain selenium in a weightfraction of 40%-95% based on the total weight of the non-active material(e.g. conductive additive, binder, etc.) and selenium combined. Thecathode layer may further accommodate a second element selected from Sn,Sb, Bi, S, Te, or a combination thereof and the weight of the secondelement is less than the weight of selenium. The second element may bemixed with selenium (Se) to form a mixture or alloy. The second element,the mixture, or the alloy may be preferably in a nanoparticle ornanocoating form having a diameter or thickness from 0.5 nm to 100 nm.The cathode active material layer may be supported on a cathode currentcollector.

The anode active material layer may contain, as an anode activematerial, lithium metal, sodium metal, potassium metal, an alloythereof, a compound thereof, or a combination thereof. The anode layermay contain a material (e.g. graphite, hard carbon, Si, etc.) that iscapable of intercalating/de-intercalating Li, Na, or K ions. There canbe a conductive additive, binder, current collector, etc. as will beappreciated by a skilled person in the art.

The invention also includes a process for producing the grapheneseparator layer. In certain embodiments, the process comprises: (a)preparing a graphene dispersion having multiple graphene sheetsdispersed in a liquid medium, wherein the graphene sheets are selectedfrom a pristine graphene or a non-pristine graphene material, having acontent of non-carbon elements greater than 2% by weight, selected fromgraphene oxide, reduced graphene oxide, graphene fluoride, graphenechloride, graphene bromide, graphene iodide, hydrogenated graphene,nitrogenated graphene, chemically functionalized graphene, or acombination thereof and wherein said graphene dispersion contains anoptional blowing agent having a blowing agent-to-graphene materialweight ratio from 0/1.0 to 1.0/1.0; (b) dispensing and depositing thegraphene dispersion onto a surface of a supporting substrate to form awet layer of graphene; (c) partially or completely removing the liquidmedium from the wet layer of graphene to form a dried layer of graphene;and (d) heat treating the dried layer of graphene at a first heattreatment temperature selected from 80° C. to 3,200° C. at a desiredheating rate sufficient to induce volatile gas molecules from thenon-carbon elements or to activate the blowing agent for producing asheet or roll of solid graphene foam having multiple pores (cells) andpore walls (cell walls) containing graphene sheets. The dispensing anddepositing procedure in step (b) may include subjecting the graphenedispersion to an orientation-inducing stress.

In certain embodiments, the process further includes a step ofheat-treating the solid graphene foam at a second heat treatmenttemperature higher than the first heat treatment temperature for alength of time sufficient for increasing the thermal conductivity of thesolid graphene foam wherein the pore walls contain stacked grapheneplanes having an inter-plane spacing d₀₀₂ from 0.3354 nm to 0.36 nm anda content of non-carbon elements less than 2% by weight.

Some details about how to prepare graphene dispersion in step (a) of theinvented process are presented below. The graphite intercalationcompound (GIC) or graphite oxide may be obtained by immersing powders orfilaments of a starting graphitic material in an intercalating/oxidizingliquid medium (e.g. a mixture of sulfuric acid, nitric acid, andpotassium permanganate) in a reaction vessel. The starting graphiticmaterial may be selected from natural graphite, artificial graphite,mesophase carbon, mesophase pitch, mesocarbon microbead, soft carbon,hard carbon, coke, carbon fiber, carbon nanofiber, carbon nanotube, or acombination thereof.

When the starting graphite powders or filaments are mixed in theintercalating/oxidizing liquid medium, the resulting slurry is aheterogeneous suspension and appears dark and opaque. When the oxidationof graphite proceeds at a reaction temperature for a sufficient lengthof time (4-120 hours at room temperature, 20-25° C.), the reacting masscan eventually become a suspension that appears slightly green andyellowish, but remain opaque. If the degree of oxidation is sufficientlyhigh (e.g. having an oxygen content between 20% and 50% by weight,preferably between 30% and 50%) and all the original graphene planes arefully oxidized, exfoliated and separated to the extent that eachoxidized graphene plane (now a graphene oxide sheet or molecule) issurrounded by the molecules of the liquid medium, one obtains a GO gel.

The aforementioned features are further described and explained indetail as follows: As illustrated in FIG. 1, a graphite particle (e.g.100) is typically composed of multiple graphite crystallites or grains.A graphite crystallite is made up of layer planes of hexagonal networksof carbon atoms. These layer planes of hexagonally arranged carbon atomsare substantially flat and are oriented or ordered so as to besubstantially parallel and equidistant to one another in a particularcrystallite. These layers of hexagonal-structured carbon atoms, commonlyreferred to as graphene layers or basal planes, are weakly bondedtogether in their thickness direction (crystallographic c-axisdirection) by weak van der Waals forces and groups of these graphenelayers are arranged in crystallites. The graphite crystallite structureis usually characterized in terms of two axes or directions: the c-axisdirection and the a-axis (or b-axis) direction. The c-axis is thedirection perpendicular to the basal planes. The a- or b-axes are thedirections parallel to the basal planes (perpendicular to the c-axisdirection).

A highly ordered graphite particle can consist of crystallites of aconsiderable size, having a length of L_(a) along the crystallographica-axis direction, a width of L_(b) along the crystallographic b-axisdirection, and a thickness L_(c) along the crystallographic c-axisdirection. The constituent graphene planes of a crystallite are highlyaligned or oriented with respect to each other and, hence, theseanisotropic structures give rise to many properties that are highlydirectional. For instance, the thermal and electrical conductivity of acrystallite are of great magnitude along the plane directions (a- orb-axis directions), but relatively low in the perpendicular direction(c-axis). As illustrated in the upper-left portion of FIG. 1, differentcrystallites in a graphite particle are typically oriented in differentdirections and, hence, a particular property of a multi-crystallitegraphite particle is the directional average value of all theconstituent crystallites.

Due to the weak van der Waals forces holding the parallel graphenelayers, natural graphite can be treated so that the spacing between thegraphene layers can be appreciably opened up so as to provide a markedexpansion in the c-axis direction, and thus form an expanded graphitestructure in which the laminar character of the carbon layers issubstantially retained. The process for manufacturing flexible graphiteis well-known in the art. In general, flakes of natural graphite (e.g.100 in FIG. 1) are intercalated in an acid solution to produce graphiteintercalation compounds (GICs, 102). The GICs are washed, dried, andthen exfoliated by exposure to a high temperature for a short period oftime. This causes the flakes to expand or exfoliate in the c-axisdirection of the graphite up to 80-300 times of their originaldimensions. The exfoliated graphite flakes are vermiform in appearanceand, hence, are commonly referred to as worms 104. These worms ofgraphite flakes which have been greatly expanded can be formed withoutthe use of a binder into cohesive or integrated sheets of expandedgraphite, e.g. webs, papers, strips, tapes, foils, mats or the like(typically referred to as “flexible graphite” 106) having a typicaldensity of about 0.04-2.0 g/cm³ for most applications.

In one prior art process, the exfoliated graphite (or mass of graphiteworms) is re-compressed by using a calendaring or roll-pressingtechnique to obtain flexible graphite foils (106 in FIG. 1), which aretypically 100-300 μm thick.

Largely due to the presence of defects, commercially available flexiblegraphite foils normally have an in-plane electrical conductivity of1,000-3,000 S/cm, through-plane (thickness-direction or Z-direction)electrical conductivity of 15-30 S/cm, in-plane thermal conductivity of140-300 W/mK, and through-plane thermal conductivity of approximately10-30 W/mK. These defects are also responsible for the low mechanicalstrength (e.g. defects are potential stress concentration sites wherecracks are preferentially initiated). These properties are inadequatefor many thermal management applications and the present invention ismade to address these issues. In another prior art process, theexfoliated graphite worm may be impregnated with a resin and thencompressed and cured to form a flexible graphite composite, which isnormally of low strength as well. In addition, upon resin impregnation,the electrical and thermal conductivity of the graphite worms could bereduced by two orders of magnitude.

Alternatively, the exfoliated graphite may be subjected tohigh-intensity mechanical shearing/separation treatments using ahigh-intensity air jet mill, high-intensity ball mill, or ultrasonicdevice to produce separated nano graphene platelets (NGPs) with all thegraphene platelets thinner than 100 nm, mostly thinner than 10 nm, and,in many cases, being single-layer graphene (also illustrated as 112 inFIG. 1). An NGP is composed of a graphene sheet or a plurality ofgraphene sheets with each sheet being a two-dimensional, hexagonalstructure of carbon atoms.

Further alternatively, with a low-intensity shearing, graphite wormstend to be separated into the so-called expanded graphite flakes (108 inFIG. 1) having a thickness >100 nm. These flakes can be formed intographite paper or mat 106 using a paper- or mat-making process. Thisexpanded graphite paper or mat 106 is just a simple aggregate or stackof discrete flakes having defects, interruptions, and mis-orientationsbetween these discrete flakes.

For the purpose of defining the geometry and orientation of an NGP, theNGP is described as having a length (the largest dimension), a width(the second largest dimension), and a thickness. The thickness is thesmallest dimension, which is no greater than 100 nm, preferably smallerthan 10 nm and most preferably 0.34 nm-1.7 nm in the presentapplication. When the platelet is approximately circular in shape, thelength and width are referred to as diameter. In the presently definedNGPs, both the length and width can be smaller than 1 μm, but can belarger than 200 μm.

A mass of multiple NGPs (including discrete sheets/platelets ofsingle-layer and/or few-layer graphene or graphene oxide) may be readilydispersed in water or a solvent and then made into a graphene paper (114in FIG. 1) using a paper-making process. Many discrete graphene sheetsare folded or interrupted (not integrated), most of plateletorientations being not parallel to the paper surface. The existence ofmany defects or imperfections leads to poor electrical and thermalconductivity in both the in-plane and the through-plane (thickness-)directions.

Fluorinated graphene or graphene fluoride is herein used as an exampleof the halogenated graphene material group. There are two differentapproaches that have been followed to produce fluorinated graphene: (1)fluorination of pre-synthesized graphene: This approach entails treatinggraphene prepared by mechanical exfoliation or by CVD growth withfluorinating agent such as XeF₂, or F-based plasmas; (2) Exfoliation ofmultilayered graphite fluorides: Both mechanical exfoliation and liquidphase exfoliation of graphite fluoride can be readily accomplished [F.Karlicky, et al. “Halogenated Graphenes: Rapidly Growing Family ofGraphene Derivatives” ACS Nano, 2013, 7 (8), pp 6434-6464].

Interaction of F₂ with graphite at high temperature leads to covalentgraphite fluorides (CF)_(n) or (C₂F)_(n), while at low temperaturesgraphite intercalation compounds (GIC) C_(x)F (2≤x≤24) form. In (CF)_(n)carbon atoms are sp3-hybridized and thus the fluorocarbon layers arecorrugated consisting of trans-linked cyclohexane chairs. In (C₂F)_(n)only half of the C atoms are fluorinated and every pair of the adjacentcarbon sheets are linked together by covalent C—C bonds. Systematicstudies on the fluorination reaction showed that the resulting F/C ratiois largely dependent on the fluorination temperature, the partialpressure of the fluorine in the fluorinating gas, and physicalcharacteristics of the graphite precursor, including the degree ofgraphitization, particle size, and specific surface area. In addition tofluorine (F₂), other fluorinating agents may be used, although most ofthe available literature involves fluorination with F₂ gas, sometimes inpresence of fluorides.

For exfoliating a layered precursor material to the state of individualsingle graphene layers or few-layers, it is necessary to overcome theattractive forces between adjacent layers and to further stabilize thelayers. This may be achieved by either covalent modification of thegraphene surface by functional groups or by non-covalent modificationusing specific solvents, surfactants, polymers, or donor-acceptoraromatic molecules. The process of liquid phase exfoliation includesultra-sonic treatment of a graphite fluoride in a liquid medium toproduce graphene fluoride sheets dispersed in the liquid medium. Theresulting dispersion can be directly made into a sheet of paper or aroll of paper.

The nitrogenation of graphene can be conducted by exposing a graphenematerial, such as graphene oxide, to ammonia at high temperatures(200-400° C.). Nitrogenated graphene could also be formed at lowertemperatures by a hydrothermal method; e.g. by sealing GO and ammonia inan autoclave and then increased the temperature to 150-250° C. Othermethods to synthesize nitrogen doped graphene include nitrogen plasmatreatment on graphene, arc-discharge between graphite electrodes in thepresence of ammonia, ammonolysis of graphene oxide under CVD conditions,and hydrothermal treatment of graphene oxide and urea at differenttemperatures.

For the purpose of defining the claims of the instant application, NGPsor graphene materials include discrete sheets/platelets of single-layerand multi-layer (typically less than 10 layers, the few-layer graphene)pristine graphene, graphene oxide, reduced graphene oxide (RGO),graphene fluoride, graphene chloride, graphene bromide, graphene iodide,hydrogenated graphene, nitrogenated graphene, chemically functionalizedgraphene, doped graphene (e.g. doped by B or N). Pristine graphene hasessentially 0% oxygen. RGO typically has an oxygen content of 0.001%-5%by weight. Graphene oxide (including RGO) can have 0.001%-50% by weightof oxygen. Other than pristine graphene, all the graphene materials have0.001%-50% by weight of non-carbon elements (e.g. O, H, N, B, F, Cl, Br,I, etc.). These materials are herein referred to as non-pristinegraphene materials. The presently invented graphene-carbon foam cancontain pristine or non-pristine graphene and the invented method allowsfor this flexibility.

Briefly, in certain embodiments, the process for producing the inventedsolid graphene foam (e.g. in a layer form) comprises the followingsteps:

(a) preparing a graphene dispersion having sheets or molecules of agraphene material dispersed in a liquid medium, wherein the graphenematerial is selected from pristine graphene, graphene oxide, reducedgraphene oxide, graphene fluoride, graphene chloride, graphene bromide,graphene iodide, hydrogenated graphene, nitrogenated graphene,chemically functionalized graphene, or a combination thereof and whereinthe dispersion contains an optional blowing agent with a blowingagent-to-graphene material weight ratio from 0/1.0 to 1.0/1.0 (thisblowing agent is normally required if the graphene material is pristinegraphene, typically having a blowing agent-to-pristine graphene weightratio from 0.01/1.0 to 1.0/1.0);(b) dispensing and depositing the graphene dispersion onto a surface ofa supporting substrate (e.g. plastic film, rubber sheet, metal foil,glass sheet, paper sheet, etc.) to form a wet layer of graphene-anodematerial mixture, wherein the dispensing and depositing procedure (e.g.coating or casting) preferably includes subjecting the graphenedispersion to an orientation-inducing stress (e.g. via slot-die coating,comma coating, reverse-roll coating, casting; etc.);(c) partially or completely removing the liquid medium from the wetlayer of graphene material to form a dried layer of material mixture,with the graphene material having a content of non-carbon elements (e.g.O, H, N, B, F, Cl, Br, I, etc.) no less than 5% by weight (thisnon-carbon content, when being removed via heat-induced decomposition,produces volatile gases that act as a foaming agent or blowing agent);and(d) heat treating the dried layer of material mixture at a first heattreatment temperature from 100° C. to 3,000° C. at a desired heatingrate sufficient to induce volatile gas molecules from the non-carbonelements in the graphene material or to activate the blowing agent forproducing the solid graphene foam. The graphene foam typically has adensity from 0.01 to 1.7 g/cm³ (more typically from 0.1 to 1.5 g/cm³,and even more typically from 0.1 to 1.0 g/cm³, and most typically from0.2 to 0.75 g/cm³), or a specific surface area from 50 to 3,000 m²/g(more typically from 200 to 2,000 m²/g, and most typically from 500 to1,500 m²/g).

The pores in the graphene foam are formed slightly before, during, orafter sheets of a graphene material are (1) chemically linked/mergedtogether (edge-to-edge and/or face-to-face) typically at a temperaturefrom 100 to 1,500° C. and/or (2) re-organized into larger graphitecrystals or domains (herein referred to as re-graphitization) along thepore walls at a high temperature (typically >2,100° C. and moretypically >2,500° C.). Pores are formed due to the evolution of volatilegases (from a blowing agent and/or non-carbon elements, such as —OH, —F,etc.) during the heat treatment of the dried graphene layer.

The presently invented solid graphene foam can be prepared such that itexhibits not only a controllable porosity and density, but alsoexcellent elasticity. In particular, the solid graphene foam inaccordance with the invention surprisingly can exhibit a low compressionset value (for example less than 15%) when compressed 80% or more of itsoriginal volume, or a compression set less than 10% when compressed 50%or more of its original volume. Such a high elasticity property enablesthe graphene separator layer to maintain good physical contact with thecathode active material layer and, as such, the graphene separator layerappears to be more effective in eliminating or reducing the shuttleeffect of the battery. The ability of the pore walls to snap back uponrelease of a mechanical stress exerted on this type of graphene foamlikely originates from the graphene sheets that are bonded and joint toform larger and stronger graphene planes during heat treatments. Aplausible mechanism may be illustrated in FIG. 3.

A blowing agent or foaming agent is a substance which is capable ofproducing a cellular or foamed structure via a foaming process in avariety of materials that undergo hardening or phase transition, such aspolymers (plastics and rubbers), glass, and metals. They are typicallyapplied when the material being foamed is in a liquid state. It has notbeen previously known that a blowing agent can be used to create afoamed material while in a solid state. More significantly, it has notbeen taught or hinted that an aggregate of sheets of a graphene materialcan be converted into a graphene foam via a blowing agent. The cellularstructure in a matrix is typically created for the purpose of reducingdensity, increasing thermal resistance and acoustic insulation, whileincreasing the thickness and relative stiffness of the original polymer.

Blowing agents or related foaming mechanisms to create pores or cells(bubbles) in a matrix for producing a foamed or cellular material, canbe classified into the following groups:

-   -   (a) Physical blowing agents: e.g. hydrocarbons (e.g. pentane,        isopentane, cyclopentane), chlorofluorocarbons (CFCs),        hydrochlorofluorocarbons (HCFCs), and liquid CO₂. The        bubble/foam-producing process is endothermic, i.e. it needs heat        (e.g. from a melt process or the chemical exotherm due to        cross-linking), to volatize a liquid blowing agent.    -   (b) Chemical blowing agents: e.g. isocyanate, azo-, hydrazine        and other nitrogen-based materials (for thermoplastic and        elastomeric foams), sodium bicarbonate (e.g. baking soda, used        in thermoplastic foams). Here gaseous products and other        by-products are formed by a chemical reaction, promoted by        process or a reacting polymer's exothermic heat. Since the        blowing reaction involves forming low molecular weight compounds        that act as the blowing gas, additional exothermic heat is also        released. Powdered titanium hydride is used as a foaming agent        in the production of metal foams, as it decomposes to form        titanium and hydrogen gas at elevated temperatures.        Zirconium (II) hydride is used for the same purpose. Once formed        the low molecular weight compounds will never revert to the        original blowing agent(s), i.e. the reaction is irreversible.    -   (c) Mixed physical/chemical blowing agents: e.g. used to produce        flexible polyurethane (PU) foams with very low densities. Both        the chemical and physical blowing can be used in tandem to        balance each other out with respect to thermal energy        released/absorbed; hence, minimizing temperature rise. For        instance, isocyanate and water (which react to form CO₂) are        used in combination with liquid CO₂ (which boils to give gaseous        form) in the production of very low density flexible PU foams        for mattresses.    -   (d) Mechanically injected agents: Mechanically made foams        involve methods of introducing bubbles into liquid polymerizable        matrices (e.g. an unvulcanized elastomer in the form of a liquid        latex). Methods include whisking-in air or other gases or low        boiling volatile liquids in low viscosity lattices, or the        injection of a gas into an extruder barrel or a die, or into        injection molding barrels or nozzles and allowing the shear/mix        action of the screw to disperse the gas uniformly to form very        fine bubbles or a solution of gas in the melt. When the melt is        molded or extruded and the part is at atmospheric pressure, the        gas comes out of solution expanding the polymer melt immediately        before solidification.    -   (e) Soluble and leachable agents: Soluble fillers, e.g. solid        sodium chloride crystals mixed into a liquid urethane system,        which is then shaped into a solid polymer part, the sodium        chloride is later washed out by immersing the solid molded part        in water for some time, to leave small inter-connected holes in        relatively high density polymer products.    -   (f) We have found that the above five mechanisms can all be used        to create pores in the graphene materials while they are in a        solid state. Another mechanism of producing pores in a graphene        material is through the generation and vaporization of volatile        gases by removing those non-carbon elements in a        high-temperature environment. This is a unique self-foaming        process that has never been previously taught or suggested.

FIG. 2(B) provides a schematic drawing to illustrate an example of apaper-making operation (using a mold cavity cell with a vacuum-assistedsuction provision) for forming a graphene paper layer of compacted andoriented graphene sheets 326. The process begins with dispersingisolated graphene sheets 322 and an optional conductive filler in aliquid medium 324 to form a dispersion. This is followed by generating anegative pressure via a vacuum system that sucks excess liquid 332through channels 330. This operation acts to reduce the dispersionvolume and align all the isolated graphene sheets on the bottom plane ofa mold cavity cell. Compacted graphene sheets are aligned parallel tothe bottom plane or perpendicular to the layer thickness direction.Optionally, the resulting layer of laminar graphene paper structure maybe further compressed to achieve an even high tap density. The sameprocedure may be used to produce carbon or graphite paper from, forinstance, carbon fibers, carbon nanofibers, carbon nanotubes, etc.

Example 1: Various Blowing Agents and Pore-Forming (Bubble-Producing)Processes

In the field of plastic processing, chemical blowing agents are mixedinto the plastic pellets in the form of powder or pellets and dissolvedat higher temperatures. Above a certain temperature specific for blowingagent dissolution, a gaseous reaction product (usually nitrogen or CO₂)is generated, which acts as a blowing agent. However, a chemical blowingagent cannot be dissolved in a graphene material, which is a solid, notliquid. This presents a challenge to make use of a chemical blowingagent to generate pores or cells in a graphene material.

After extensive experimenting, we have discovered that practically anychemical blowing agent (e.g. in a powder or pellet form) can be used tocreate pores or bubbles in a dried layer of graphene when the first heattreatment temperature is sufficient to activate the blowing reaction.The chemical blowing agent (powder or pellets) may be dispersed in theliquid medium to become a second dispersed phase (sheets of graphenematerial being the first dispersed phase) in the suspension, which canbe deposited onto the solid supporting substrate to form a wet layer.This wet layer of graphene material may then be dried and heat treatedto activate the chemical blowing agent. After a chemical blowing agentis activated and bubbles are generated, the resulting foamed graphenestructure is largely maintained even when subsequently a higher heattreatment temperature is applied to the structure. This is quiteunexpected, indeed.

Chemical foaming agents (CFAs) can be organic or inorganic compoundsthat release gasses upon thermal decomposition. CFAs are typically usedto obtain medium- to high-density foams, and are often used inconjunction with physical blowing agents to obtain low-density foams.CFAs can be categorized as either endothermic or exothermic, whichrefers to the type of decomposition they undergo. Endothermic typesabsorb energy and typically release carbon dioxide and moisture upondecomposition, while the exothermic types release energy and usuallygenerate nitrogen when decomposed. The overall gas yield and pressure ofgas released by exothermic foaming agents is often higher than that ofendothermic types. Endothermic CFAs are generally known to decompose inthe range of 130 to 230° C. (266-446° F.), while some of the more commonexothermic foaming agents decompose around 200° C. (392° F.). However,the decomposition range of most exothermic CFAs can be reduced byaddition of certain compounds. The activation (decomposition)temperatures of CFAs fall into the range of our heat treatmenttemperatures. Examples of suitable chemical blowing agents includesodium bicarbonate (baking soda), hydrazine, hydrazide, azodicarbonamide(exothermic chemical blowing agents), nitroso compounds (e.g. N,N-dinitroso pentamethylene tetramine), hydrazine derivatives (e.g. 4.4′-oxybis (benzenesulfonyl hydrazide) and hydrazo dicarbonamide), andhydrogen carbonate (e.g. sodium hydrogen carbonate). These are allcommercially available in plastics industry.

In the production of foamed plastics, physical blowing agents aremetered into the plastic melt during foam extrusion or injection moldedfoaming, or supplied to one of the precursor materials duringpolyurethane foaming. It has not been previously known that a physicalblowing agent can be used to create pores in a graphene material, whichis in a solid state (not melt). We have surprisingly observed that aphysical blowing agent (e.g. CO₂ or N₂) can be injected into the streamof graphene suspension prior to being coated or cast onto the supportingsubstrate. This would result in a foamed structure even when the liquidmedium (e.g. water and/or alcohol) is removed. The dried layer ofgraphene material is capable of maintaining a controlled amount of poresor bubbles during liquid removal and subsequent heat treatments.

Technically feasible blowing agents include carbon dioxide (CO₂),nitrogen (N₂), isobutane (C₄H₁₀), cyclopentane (C₅H₁₀), isopentane(C₅H₁₂), CFC-11 (CFCI₃), HCFC-22 (CHF₂CI), HCFC-142b (CF₂CICH₃),HCFC-134a (CH₂FCF₃), isobutane and pentane.

Except for the regulated CFC substances, all the blowing agents recitedabove have been tested in our experiments. For both physical blowingagents and chemical blowing agents, the blowing agent amount introducedinto the suspension is defined as a blowing agent-to-graphene materialweight ratio, which is typically from 0/1.0 to 1.0/1.0.

Example 2: Preparation of Solid Graphene Foam from Graphene Oxide Sheets

Chopped graphite fibers with an average diameter of 12 μm and naturalgraphite particles were separately used as a starting material, whichwas immersed in a mixture of concentrated sulfuric acid, nitric acid,and potassium permanganate (as the chemical intercalate and oxidizer) toprepare graphite intercalation compounds (GICs). The starting materialwas first dried in a vacuum oven for 24 h at 80° C. Then, a mixture ofconcentrated sulfuric acid, fuming nitric acid, and potassiumpermanganate (at a weight ratio of 4:1:0.05) was slowly added, underappropriate cooling and stirring, to a three-neck flask containing fibersegments. After 5-16 hours of reaction, the acid-treated graphite fibersor natural graphite particles were filtered and washed thoroughly withdeionized water until the pH level of the solution reached 6. Afterbeing dried at 100° C. overnight, the resulting graphite intercalationcompound (GIC) or graphite oxide fiber was re-dispersed in water and/oralcohol to form a slurry.

In one sample, five grams of the graphite oxide fibers were mixed with2,000 ml alcohol solution consisting of alcohol and distilled water witha ratio of 15:85 to obtain a slurry mass. Then, the mixture slurry wassubjected to ultrasonic irradiation with a power of 200 W for variouslengths of time. After 20 minutes of sonication, GO fibers wereeffectively exfoliated and separated into thin graphene oxide sheetswith oxygen content of approximately 23%-31% by weight. The resultingsuspension contains GO sheets being suspended in water. A chemicalblowing agent (hydrazo dicarbonamide) was added to the suspension justprior to casting.

The resulting suspension was then cast onto a glass surface using adoctor's blade to exert shear stresses, inducing GO sheet orientations.The resulting GO coating films, after removal of liquid, have athickness that can be varied from approximately 5 to 500 μm (preferablyand typically from 10 μm to 50 μm).

For making a graphene foam specimen, the GO coating film was thensubjected to heat treatments that typically involve an initial thermalreduction temperature of 80-350° C. for 1-8 hours, followed byheat-treating at a second temperature of 1,500-2,850° C. for 0.5 to 5hours. It may be noted that we have found it essential to apply acompressive stress to the coating film sample while being subjected tothe first heat treatment. This compress stress seems to have helpedmaintain good contacts between the graphene sheets so that chemicalmerging and linking between graphene sheets can occur while pores arebeing formed. Without such a compressive stress, the heat-treated filmis typically excessively porous with constituent graphene sheets in thepore walls being very poorly oriented and incapable of chemical mergingand linking with one another. As a result, the thermal conductivity,electrical conductivity, and mechanical strength of the graphene foamare severely compromised. Shown in FIG. 4 are the in-plane andthrough-plane electrical conductivity values of the GO-derived graphenefoam sheets (prepared by Comma coating, heat treatment, andcompression).

The solid graphene foam typically has a high degree of elasticity (fullyrecoverable elastic deformation) as reflected by a low permanentcompression set (compression deformation that is not recoverable). Thesolid graphene foam typically has a compression set (at 15% compression)of 15% or less and, in many cases, 8% or less. Many specimens have acompression set (at 50% compression) of 10% or less and, in many cases,5% or less.

The compression set measurement was conducted according to ASTM D395.The measured value of “compression set” is expressed as the percentageof the original deflection (i.e. a constant deflection test). A testspecimen of the solid graphene foam was compressed at a nominated % forone minute at 25° C. Compression set was taken as the % of the originaldeflection after the specimen was allowed to recover at standardconditions for 30 minutes. The compression set value C can be calculatedusing the formula [(t₀−t_(i))/(t₀−t_(n))]×100, where t₀ is the originalspecimen thickness, t_(i) the specimen thickness after testing, andt_(n) is the spacer thickness which sets the % compression that the foamis to be subjected. For comparative results, the specimens tested allhad the same dimensions: diameter of about 12 mm and height of about 8mm.

Example 3: Preparation of Single-Layer Graphene Sheets from MesocarbonMicrobeads (MCMBs)

Mesocarbon microbeads (MCMBs) were supplied from China Steel ChemicalCo., Kaohsiung, Taiwan. This material has a density of about 2.24 g/cm³with a median particle size of about 16 μm. MCMB (10 grams) wereintercalated with an acid solution (sulfuric acid, nitric acid, andpotassium permanganate at a ratio of 4:1:0.05) for 48-96 hours. Uponcompletion of the reaction, the mixture was poured into deionized waterand filtered. The intercalated MCMBs were repeatedly washed in a 5%solution of HCl to remove most of the sulfate ions. The sample was thenwashed repeatedly with deionized water until the pH of the filtrate wasno less than 4.5. The slurry was then subjected ultrasonication for10-100 minutes to produce GO suspensions. TEM and atomic forcemicroscopic studies indicate that most of the GO sheets weresingle-layer graphene when the oxidation treatment exceeded 72 hours,and 2- or 3-layer graphene when the oxidation time was from 48 to 72hours.

The GO sheets contain oxygen proportion of approximately 35%-47% byweight for oxidation treatment times of 48-96 hours. GO sheets weresuspended in water. Baking soda (5-20% by weight), as a chemical blowingagent, was added to the suspension just prior to casting. The GOsuspension was then cast onto a glass surface using a doctor's blade toexert shear stresses, inducing GO sheet orientations. Several sampleswere cast, some containing a blowing agent and some not. The resultingGO films, after removal of liquid, have a thickness that can be variedfrom approximately 10 to 500 μm.

The several sheets of the GO film, with or without a blowing agent, werethen subjected to heat treatments that involve an initial (first)thermal reduction temperature of 80-500° C. for 1-5 hours. This firstheat treatment generated a graphene foam. However, the graphene domainsin the foam wall can be further perfected (re-graphitized to become moreordered or having a higher degree of crystallinity and larger lateraldimensions of graphene planes, longer than the original graphene sheetdimensions due to chemical merging) if the foam is followed byheat-treating at a second temperature of 1,500-2,850° C.

The solid graphene foam produced in this manner typically is open-cellfoam containing interconnected cells. However, the heat treatments canenable graphene sheet merging or chemical bonding with one another,resulting in a foamed structure that has adequate elasticity andconductivity.

Example 4: Preparation of Pristine Graphene Foam (0% Oxygen)

Recognizing the possibility of the high defect population in GO sheetsacting to reduce the conductivity of individual graphene plane, wedecided to study if the use of pristine graphene sheets (non-oxidizedand oxygen-free, non-halogenated and halogen-free, etc.) can lead to agraphene foam having a higher thermal conductivity. Pristine graphenesheets were produced by using the direct ultrasonication or liquid-phaseproduction process.

In a typical procedure, five grams of graphite flakes, ground toapproximately 20 μm or less in sizes, were dispersed in 1,000 mL ofdeionized water (containing 0.1% by weight of a dispersing agent, Zonyl®FSO from DuPont) to obtain a suspension. An ultrasonic energy level of85 W (Branson 5450 Ultrasonicator) was used for exfoliation, separation,and size reduction of graphene sheets for a period of 15 minutes to 2hours. The resulting graphene sheets are pristine graphene that havenever been oxidized and are oxygen-free and relatively defect-free.There are no other non-carbon elements.

Various amounts (1%-30% by weight relative to graphene material) ofchemical bowing agents (N, N-dinitroso pentamethylene tetramine or 4.4′-oxybis (benzenesulfonyl hydrazide) were added to a suspensioncontaining pristine graphene sheets and a surfactant. The suspension wasthen cast onto a glass surface using a doctor's blade to exert shearstresses, inducing graphene sheet orientations. Several samples werecast, including one that was made using CO₂ as a physical blowing agentintroduced into the suspension just prior to casting). The resultinggraphene films, after removal of liquid, have a thickness that can bevaried from approximately 10 to 100 μm.

The graphene films were then subjected to heat treatments that involvean initial (first) thermal reduction temperature of 80-1,500° C. for 1-5hours. This first heat treatment led to the production of a graphenefoam. Some of the pristine foam samples were then subjected to a secondtemperature of 1,500-2,850° C. to determine if the graphene domains inthe foam wall could be further perfected (re-graphitized to become moreordered or having a higher degree of crystallinity).

The solid graphene foam typically has a compression set (at 15%compression) of 15% or less and, in many cases, 8% or less. Manyspecimens have a compression set (at 50% compression) of 10% or lessand, in many cases, 5% or less.

Example 5: CVD Graphene Foams on Ni Foam Templates

The procedure was adapted from that disclosed in open literature: Chen,Z. et al. “Three-dimensional flexible and conductive interconnectedgraphene networks grown by chemical vapor deposition,” Nat. Mater. 10,424-428 (2011). Nickel foam, a porous structure with an interconnected3D scaffold of nickel was chosen as a template for the growth ofgraphene foam. Briefly, carbon was introduced into a nickel foam bydecomposing CH₄ at 1,000° C. under ambient pressure, and graphene filmswere then deposited on the surface of the nickel foam. Due to thedifference in the thermal expansion coefficients between nickel andgraphene, ripples and wrinkles were formed on the graphene films. Inorder to recover (separate) graphene foam, Ni frame must be etched away.Before etching away the nickel skeleton by a hot HCl (or FeCl₃)solution, a thin layer of poly(methyl methacrylate) (PMMA) was depositedon the surface of the graphene films as a support to prevent thegraphene network from collapsing during nickel etching. After the PMMAlayer was carefully removed by hot acetone, a fragile graphene foamsample was obtained. The use of the PMMA support layer is critical topreparing a free-standing film of graphene foam; only a severelydistorted and deformed graphene foam sample was obtained without thePMMA support layer. This is a tedious process that is notenvironmentally benign and is not scalable.

Example 6: Conventional Graphitic Foam from Pitch-Based Carbon Foams

Pitch powder, granules, or pellets are placed in a aluminum mold withthe desired final shape of the foam. Mitsubishi ARA-24 mesophase pitchwas utilized. The sample is evacuated to less than 1 torr and thenheated to a temperature approximately 300° C. At this point, the vacuumwas released to a nitrogen blanket and then a pressure of up to 1,000psi was applied. The temperature of the system was then raised to 800°C. This was performed at a rate of 2 degree C./min. The temperature washeld for at least 15 minutes to achieve a soak and then the furnacepower was turned off and cooled to room temperature at a rate ofapproximately 1.5 degree C./min with release of pressure at a rate ofapproximately 2 psi/min. Final foam temperatures were 630° C. and 800°C. During the cooling cycle, pressure is released gradually toatmospheric conditions. The foam was then heat treated to 1050° C.(carbonized) under a nitrogen blanket and then heat treated in separateruns in a graphite crucible to 2500° C. and 2800° C. (graphitized) inArgon.

Samples from this conventional graphitic foam were machined intospecimens for measuring the thermal conductivity. The bulk thermalconductivity of the graphitic foam was found to be in the range from 67W/mK to 151 W/mK. The density of the samples was from 0.31 to 0.61g/cm³. When the material porosity level is taken into account, thespecific thermal conductivity of the mesophase pitch derived foam isapproximately 67/0.31=216 and 151/0.61=247.5 W/mK per specific gravity(or per physical density). In contrast, the specific thermalconductivity of the presently invented foam is typically >>250 W/mK perspecific gravity.

The compression strength of the conventional graphitic foam sampleshaving an average density of 0.51 g/cm³ was measured to be 3.6 MPa andthe compression modulus was measured to be 74 MPa. By contrast, thecompression strength and compressive modulus of the presently inventedgraphene foam samples derived from GO having a comparable physicaldensity are 5.7 MPa and 103 MPa, respectively.

Shown in FIG. 5(A) and FIG. 6(A) are the thermal conductivity values vs.specific gravity of the GO suspension-derived foam (Example 3),mesophase pitch-derived graphite foam (Comparative Example 4-b), and Nifoam template-assisted CVD graphene foam (Comparative Example 4-a).These data clearly demonstrate the following unexpected results:

-   -   1) GO-derived graphene foams produced by the presently invented        process exhibit significantly higher thermal conductivity as        compared to both mesophase pitch-derived graphite foam and Ni        foam template-assisted CVD graphene, given the same physical        density.    -   2) This higher thermal conductivity is quite surprising in view        of the notion that CVD graphene is essentially pristine graphene        that has never been exposed to oxidation and should have        exhibited a much higher thermal conductivity compared to        graphene oxide (GO). GO is known to be highly defective (having        a high defect population and, hence, low conductivity) even        after the oxygen-containing functional groups are removed via        conventional thermal or chemical reduction methods. These        exceptionally high thermal conductivity values observed with the        GO-derived graphene foams herein produced are much to our        surprise. A good thermal dissipation capability is essential to        the prevention of thermal run-away and explosion, a most serious        problem associated with rechargeable lithium-ion batteries. A        high electrical conductivity also makes it feasible for the        solid graphene foam to serve as a cathode heat dissipater, in        addition to playing the role of protecting Se or metal selenide        (e.g. preventing or reducing the out-migration of Se or lithium        polyselenide).    -   3) FIG. 6(A) presents the thermal conductivity values over        comparable ranges of specific gravity values to allow for        calculation of specific conductivity (conductivity value, W/mK,        divided by physical density value, g/cm³) for all three        graphitic foam materials based on the slopes of the curves        (approximately straight lines at different segments). These        specific conductivity values enable a fair comparison of thermal        conductivity values of these three types of graphitic foams        given the same amount of solid graphitic material in each type        of foam. These data provide an index of the intrinsic        conductivity of the solid portion of the foam material. These        data clearly indicate that, given the same amount of solid        material, the presently invented GO-derived foam is        intrinsically most conducting, reflecting a high level of        graphitic crystal perfection (larger crystal dimensions, fewer        grain boundaries and other defects, better crystal orientation,        etc.). This is also unexpected.    -   4) The specific conductivity values of the presently invented        GO- and GF-derived foam exhibit values from 250 to 500 W/mK per        unit of specific gravity; but those of the other two foam        materials are typically lower than 250 W/mK per unit of specific        gravity.

Summarized in FIG. 8 are thermal conductivity data for a series ofGO-derived graphene foams and a series of pristine graphene derivedfoams, both plotted over the final (maximum) heat treatmenttemperatures. These data indicate that the thermal conductivity of theGO foams is highly sensitive to the final heat treatment temperature(HTT). Even when the HTT is very low, clearly some type of graphenemerging or crystal perfection reactions are already activated. Thethermal conductivity increases monotonically with the final HTT. Incontrast, the thermal conductivity of pristine graphene foams remainsrelatively constant until a final HTT of approximately 2,500° C. isreached, signaling the beginning of a re-crystallization and perfectionof graphite crystals. There are no functional groups in pristinegraphene, such as —COOH in GO, that enable chemical linking of graphenesheets at relatively low HTTs. With a HTT as low as 1,250° C., GO sheetscan merge to form significantly larger graphene sheets with reducedgrain boundaries and other defects. Even though GO sheets areintrinsically more defective than pristine graphene, the presentlyinvented process enables the GO sheets to form graphene foams thatoutperform pristine graphene foams. This is another unexpected result.

Example 7: Preparation of Graphene Oxide (GO) Suspension from NaturalGraphite and Preparation of Subsequent GO Foams

Graphite oxide was prepared by oxidation of graphite flakes with anoxidizer liquid consisting of sulfuric acid, sodium nitrate, andpotassium permanganate at a ratio of 4:1:0.05 at 30° C. When naturalgraphite flakes (particle sizes of 14 μm) were immersed and dispersed inthe oxidizer mixture liquid for 48 hours, the suspension or slurryappears and remains optically opaque and dark. After 48 hours, thereacting mass was rinsed with water 3 times to adjust the pH value to atleast 3.0. A final amount of water was then added to prepare a series ofGO-water suspensions. We observed that GO sheets form a liquid crystalphase when GO sheets occupy a weight fraction >3% and typically from 5%to 15%.

By dispensing and coating the GO suspension on a polyethyleneterephthalate (PET) film in a slurry coater and removing the liquidmedium from the coated film we obtained a thin film of dried grapheneoxide. Several GO film samples were then subjected to different heattreatments, which typically include a thermal reduction treatment at afirst temperature of 100° C. to 500° C. for 1-10 hours, and at a secondtemperature of 1,500° C.-2,850° C. for 0.5-5 hours. With these heattreatments, also under a compressive stress, the GO films weretransformed into graphene foam.

Example 8: Graphene Foams from Hydrothermally Reduced Graphene Oxide

For comparison, a self-assembled graphene hydrogel (SGH) sample wasprepared by a one-step hydrothermal method. In a typical procedure, theSGH can be easily prepared by heating 2 mg/mL of homogeneous grapheneoxide (GO) aqueous dispersion sealed in a Teflon-lined autoclave at 180°C. for 12 h. The SGH containing about 2.6% (by weight) graphene sheetsand 97.4% water has an electrical conductivity of approximately 5×10⁻³S/cm. Upon drying and heat treating at 1,500° C., the resulting graphenefoam exhibits an electrical conductivity of approximately 1.5×10⁻¹ S/cm,which is 2 times lower than those of the presently invented graphenefoams produced by heat treating at the same temperature.

Example 9: Plastic Bead Template-Assisted Formation of Reduced GrapheneOxide Foams

A hard template-directed ordered assembly for a macro-porous bubbledgraphene film (MGF) was prepared. Mono-disperse poly methyl methacrylate(PMMA) latex spheres were used as the hard templates. The GO liquidcrystal prepared in Example 5 was mixed with a PMMA spheres suspension.Subsequent vacuum filtration was then conducted to prepare the assemblyof PMMA spheres and GO sheets, with GO sheets wrapped around the PMMAbeads. A composite film was peeled off from the filter, air dried andcalcinated at 800° C. to remove the PMMA template and thermally reduceGO into RGO simultaneously. The grey free-standing PMMA/GO film turnedblack after calcination, while the graphene film remained porous.

FIG. 5(B) and FIG. 6(B) show the thermal conductivity values of thepresently invented GO suspension-derived foam, GO foam produced viasacrificial plastic bead template-assisted process, and hydrothermallyreduced GO graphene foam. Most surprisingly, given the same starting GOsheets, the presently invented process produces the highest-performinggraphene foams. Electrical conductivity data summarized in FIG. 4(C) arealso consistent with this conclusion. These data further support thenotion that, given the same amount of solid material, the presentlyinvented GO suspension deposition (with stress-induced orientation) andsubsequent heat treatments give rise to a graphene foam that isintrinsically most conducting, reflecting a highest level of graphiticcrystal perfection (larger crystal dimensions, fewer grain boundariesand other defects, better crystal orientation, etc. along the porewalls).

It is of significance to point out that all the prior art processes forproducing graphite foams or graphene foams appear to providemacro-porous foams having a physical density in the range ofapproximately 0.2-0.6 g/cm³ only with pore sizes being typically toolarge (e.g. from 20 to 300 μm) for most of the intended applications. Incontrast, the instant invention provides processes that generategraphene foams having a density that can be as low as 0.01 g/cm³ and ashigh as 1.7 g/cm³. The pore sizes can be varied between mesoscaled (2-50nm) up to macro-scaled (1-500 μm) depending upon the contents ofnon-carbon elements and the amount/type of blowing agent used. Thislevel of flexibility and versatility in designing various types ofgraphene foams is unprecedented and un-matched by any prior art process.

Example 10: Preparation of Graphene Foams from Graphene Fluoride

Several processes have been used by us to produce GF, but only oneprocess is herein described as an example. In a typical procedure,highly exfoliated graphite (HEG) was prepared from intercalated compoundC₂F.xClF₃. HEG was further fluorinated by vapors of chlorine trifluorideto yield fluorinated highly exfoliated graphite (FHEG). Pre-cooledTeflon reactor was filled with 20-30 mL of liquid pre-cooled ClF₃, thereactor was closed and cooled to liquid nitrogen temperature. Then, nomore than 1 g of HEG was put in a container with holes for ClF₃ gas toaccess and situated inside the reactor. In 7-10 days a gray-beigeproduct with approximate formula C₂F was formed.

Subsequently, a small amount of FHEG (approximately 0.5 mg) was mixedwith 20-30 mL of an organic solvent (methanol, ethanol, 1-propanol,2-propanol, 1-butanol, tert-butanol, isoamyl alcohol) and subjected toan ultrasound treatment (280 W) for 30 min, leading to the formation ofhomogeneous yellowish dispersions. Five minutes of sonication was enoughto obtain a relatively homogenous dispersion, but longer sonicationtimes ensured better stability. Upon casting on a glass surface with thesolvent removed, the dispersion became a brownish film formed on theglass surface. When GF films were heat-treated, fluorine was released asgases that helped to generate pores in the film. In some samples, aphysical blowing agent (N₂ gas) was injected into the wet GF film whilebeing cast. These samples exhibit much higher pore volumes or lower foamdensities. Without using a blowing agent, the resulting graphenefluoride foams exhibit physical densities from 0.35 to 1.38 g/cm³. Whena blowing agent was used (blowing agent/GF weight ratio from 0.5/1 to0.05/1), a density from 0.02 to 0.35 g/cm³ was obtained. Typicalfluorine contents are from 0.001% (HTT=2,500° C.) to 4.7% (HTT=350° C.),depending upon the final heat treatment temperature involved.

FIG. 7 presents a comparison in thermal conductivity values of thegraphene foam samples derived from GO and GF (graphene fluoride),respectively, as a function of the specific gravity. It appears that theGF foams, in comparison with GO foams, exhibit higher thermalconductivity values at comparable specific gravity values. Both deliverimpressive heat-conducting capabilities, being the best among all knownfoamed materials. This was followed by a heat treatment at 500° C. for 2hours to produce a graphene foam separator layer.

Example 11: Preparation of Graphene Foams from Nitrogenated Graphene

Graphene oxide (GO), synthesized in Example 2, was finely ground withdifferent proportions of urea and the pelletized mixture heated in amicrowave reactor (900 W) for 30 s. The product was washed several timeswith deionized water and vacuum dried. In this method graphene oxidegets simultaneously reduced and doped with nitrogen. The productsobtained with graphene:urea mass ratios of 1:0.5, 1:1 and 1:2 aredesignated as NGO-1, NGO-2 and NGO-3 respectively and the nitrogencontents of these samples were 14.7, 18.2 and 17.5 wt % respectively asfound by elemental analysis. These nitrogenated graphene sheets remaindispersible in water. The resulting suspensions were then cast, dried,and heat-treated initially at 200-350° C. as a first heat treatmenttemperature and subsequently treated at a second temperature of 1,500°C. The resulting nitrogenated graphene foams exhibit physical densitiesfrom 0.45 to 1.28 g/cm³. Typical nitrogen contents of the foams are from0.01% (HTT=1,500° C.) to 5.3% (HTT=350° C.), depending upon the finalheat treatment temperature involved.

Example 12: Chemical Functionalization of Pristine Graphene Foam,Nitrogenated Graphene Foam, and Carbon Nanofiber Paper

For comparison, carbon nanofiber (CNF) paper was prepared by using thevacuum-assisted filtration procedure as schematically illustrated inFIG. 2(B). Specimens of pristine graphene foam, nitrogenated graphenefoam, and CNF paper prepared earlier were subjected to functionalizationby bringing these specimens in chemical contact with chemical compoundssuch as carboxylic acids, azide compound (2-azidoethanol), alkyl silane,diethylenetriamine (DETA), and chemical species containing hydroxylgroup, carboxyl group, amine group, and sulfonate group (—50₃H) in aliquid or solution form.

These chemical functionalization treatments generally result in fasterand more uniform and complete infiltration of the pores with Se using asolution deposition or chemical reaction-based deposition.

Example 13: Characterization of Various Graphene Foams and ConventionalGraphite Foam

The internal structures (crystal structure and orientation) of severaldried GO layers, and the heat-treated films at different stages of heattreatments were investigated using X-ray diffraction. The X-raydiffraction curve of natural graphite typically exhibits a peak atapproximately 20=26°, corresponds to an inter-graphene spacing (d₀₀₂) ofapproximately 0.3345 nm. Upon oxidation, the resulting GO shows an X-raydiffraction peak at approximately 20=12°, which corresponds to aninter-graphene spacing (d₀₀₂) of approximately 0.7 nm. With some heattreatment at 150° C., the dried GO compact exhibits the formation of ahump centered at 22°, indicating that it has begun the process ofdecreasing the inter-graphene spacing due to the beginning of chemicallinking and ordering processes. With a heat treatment temperature of2,500° C. for one hour, the d₀₀₂ spacing has decreased to approximately0.336, close to 0.3354 nm of a graphite single crystal.

With a heat treatment temperature of 2,750° C. for one hour, the d₀₀₂spacing is decreased to approximately to 0.3354 nm, identical to that ofa graphite single crystal. In addition, a second diffraction peak with ahigh intensity appears at 20=55° corresponding to X-ray diffraction from(004) plane. The (004) peak intensity relative to the (002) intensity onthe same diffraction curve, or the I(004)/I(002) ratio, is a goodindication of the degree of crystal perfection and preferred orientationof graphene planes. The (004) peak is either non-existing or relativelyweak, with the I(004)/I(002) ratio <0.1, for all graphitic materialsheat treated at a temperature lower than 2,800° C. The I(004)/I(002)ratio for the graphitic materials heat treated at 3,000-3,250° C. (e.g.highly oriented pyrolytic graphite, HOPG) is in the range of 0.2-0.5. Incontrast, a graphene foam prepared with a final HTT of 2,750° C. for onehour exhibits a I(004)/I(002) ratio of 0.75 and a Mosaic spread value of1.8, indicating a practically perfect graphene single crystal with agood degree of preferred orientation in the cell walls.

The “mosaic spread” value is obtained from the full width at halfmaximum of the (002) reflection in an X-ray diffraction intensity curve.This index for the degree of ordering characterizes the graphite orgraphene crystal size (or grain size), amounts of grain boundaries andother defects, and the degree of preferred grain orientation. A nearlyperfect single crystal of graphite is characterized by having a mosaicspread value of 0.2-0.4. Some of our graphene foams have a mosaic spreadvalue in this range of 0.2-0.4 when produced using a final heattreatment temperature no less than 2,500° C.

The inter-graphene spacing values of both the GO suspension-derived foamsamples obtained by heat treating at various temperatures over a widetemperature range are summarized in FIG. 9(A). Corresponding oxygencontent values in the GO suspension-derived graphene foam layer areshown in FIG. 9(B).

It is of significance to point out that a heat treatment temperature aslow as 500° C. is sufficient to bring the average inter-graphene spacingin GO sheets along the pore walls to below 0.4 nm, getting closer andcloser to that of natural graphite or that of a graphite single crystal.The beauty of this approach is the notion that this GO suspensionstrategy has enabled us to re-organize, re-orient, and chemically mergethe planar graphene oxide molecules from originally different graphiteparticles or graphene sheets into a unified structure with all thegraphene planes in cell walls now being larger in lateral dimensions(significantly larger than the length and width of the graphene planesin the original graphite particles). A potential chemical linkingmechanism is illustrated in FIG. 3. This has given rise to exceptionalelasticity (low compression set), thermal conductivity and electricalconductivity values.

Example 14: Electrochemical Behaviors of Li—Se and Na—Se Cells

Shown in FIG. 10 are charge/discharge cycling responses of three Li—Secells; one cell containing a presently invented GO-derived graphene foamseparator layer (ion-trapping layer), second cell containing a CNF-basedion-trapping layer, and third cell being free from such a conductingseparator or ion-trapping layer. In all three cells, the cathode layercontains a cathode active material prepared by ball-milling a mixture ofSe powder and carbon black powder. Clearly, the presently inventedgraphene foam separator or ion-trapping layer leads to the most stablecycling behavior given approximately the same Se amount in the cathode.The CNF-based ion trapping layer also works very well.

FIG. 11 shows the Ragone plots (cell power density vs. cell energydensity) of two Li metal-selenium cells, one containing a pristinegraphene foam separator layer and the other not. The cell that containsan open-cell foam-based separator layer exhibits consistently higherenergy density and power density values as compared to the cellcontaining no such separator. In addition, the implementation of agraphene foam separator layer also leads to a more stablecharge/discharge cycling behavior.

FIG. 12 shows the Ragone plots (cell power density vs. cell energydensity) of 2 alkali metal-selenium cells: a Na—Se cell featuring a RGOpaper-based separator layer and a similar Na—Se cell that does notcontain such a conducting separator layer. Again, the Na—Se cell thatcontains an open-cell foam-based separator exhibits a consistentlyhigher energy density and power density as compared to the cellcontaining no conducting separator.

We claim:
 1. A method of inhibiting the shuttle effect by preventingmigration of selenium or metal selenide ions from a cathode to an anodeof an alkali metal-selenium battery, said method comprising: (a)combining an anode active material layer, a cathode active materiallayer, an electrically insulating porous separator disposed between saidanode active material layer and said cathode active material layer, andelectrolyte to form an alkali metal-selenium battery cell, and (b)implementing a porous trapping layer, having a thickness from 5 nm to100 μm, between said cathode active material layer and said electricallyinsulating porous separator to trap selenium or metal selenide ions thatare dissolved in said electrolyte from said cathode active materiallayer.
 2. The method of claim 1, wherein said anode active materiallayer comprises an anode active material, selected from the groupconsisting of Li, Na, K, an alloy thereof, a compound thereof, graphite,carbon, Si, SiO, Sn, SnO₂, a transition metal oxide, and combinationsthereof, and an optional anode current collector supporting said anodeactive material layer.
 3. The method of claim 1, wherein said cathodeactive material layer comprises a selenium-containing material as acathode active material selected from the group consisting of selenium,a selenium-carbon hybrid, a selenium-graphite hybrid, aselenium-graphene hybrid, a conducting polymer-selenium hybrid, a metalselenide, a Se alloy or mixture with Sn, Sb, Bi, S, or Te, a seleniumcompound, and combinations thereof and further comprises an optionalcathode current collector supporting said cathode active material layer.4. The method of claim 1, wherein said porous trapping layer comprises agraphene separator layer containing a solid graphene foam, paper orfabric that is permeable to lithium ions or sodium ions but issubstantially non-permeable to selenium or metal selenide ions, whereinsaid graphene separator layer is disposed between said anode activematerial layer and said cathode active material layer and is in physicalcontact with said cathode active material layer but not in physicalcontact with said anode active material layer and wherein said grapheneseparator layer comprises pristine graphene sheets having less than0.01% by weight of non-carbon elements or non-pristine graphene sheetshaving 0.01% to 20% by weight of non-carbon elements, wherein saidnon-pristine graphene is selected from the group consisting of grapheneoxide, reduced graphene oxide, graphene fluoride, graphene chloride,graphene bromide, graphene iodide, hydrogenated graphene, nitrogenatedgraphene, boron-doped graphene, nitrogen-doped graphene, chemicallyfunctionalized graphene, and combinations thereof.
 5. The method ofclaim 4, wherein said solid graphene foam, paper or fabric has a densityranging from about 0.001 g/cm³ to about 1.7 g/cm³.
 6. The method ofclaim 4, wherein said solid graphene foam, paper, or fabric containspores having a size from 0.5 nm to 50 nm.
 7. The method of claim 1,wherein said porous trapping layer comprises a foam, paper or fabricstructure of a carbon or graphite material selected from a carbon orgraphite fiber, carbon or graphite nanofiber, carbon nanotube, carbonnanorod, mesophase carbon particle, mesocarbon microbead, expandedgraphite flake, needle coke, carbon black, acetylene black, activatedcarbon, a combination thereof, or a combination thereof with graphenesheets.
 8. The method of claim 7, wherein said carbon or graphitematerial is chemically functionalized to have a chemical functionalgroup attached thereto to promote trapping of selenium or metal selenideions.
 9. The method of claim 7, wherein said chemical functional groupis selected from alkyl or aryl silane, alkyl or aralkyl group, hydroxylgroup, carboxyl group, carboxylic group, amine group, sulfonate group(—SO₃H), aldehydic group, quinoidal, fluorocarbon, or a combinationthereof.
 10. The method of claim 7, wherein said chemical functionalgroup is selected from a derivative of an azide compound selected fromthe group consisting of 2-azidoethanol, 3-azidopropan-1-amine,4-(2-azidoethoxy)-4-oxobutanoic acid,2-azidoethyl-2-bromo-2-methylpropanoate, chlorocarbonate,azidocarbonate, dichlorocarbene, carbene, aryne, nitrene,(R-)-oxycarbonyl nitrenes, where R=any one of the following groups,

and combinations thereof.
 11. The method of claim 7, wherein saidchemical functional group is selected from an oxygenated group selectedfrom the group consisting of hydroxyl, peroxide, ether, keto, andaldehyde.
 12. The method of claim 7, wherein said chemical functionalgroup is selected from the group consisting of —SO₃H, —COOH, —NH₂, —OH,—R′CHOH, —CHO, —CN, —COCl, halide, —COSH, —SH, —COOR′, —SR′, —SiR′₃,—Si(—OR′—)_(y)R′_(3−y), —Si(—O—SiR′₂—)OR′, —R″, Li, AlR′₂, Hg—X, TlZ₂and Mg—X; wherein y is an integer equal to or less than 3, R′ ishydrogen, alkyl, aryl, cycloalkyl, or aralkyl, cycloaryl, orpoly(alkylether), R″ is fluoroalkyl, fluoroaryl, fluorocycloalkyl,fluoroaralkyl or cycloaryl, X is halide, and Z is carboxylate ortrifluoroacetate, and combinations thereof.
 13. The method of claim 7,wherein said chemical functional group is selected from the groupconsisting of amidoamines, polyamides, aliphatic amines, modifiedaliphatic amines, cycloaliphatic amines, aromatic amines, anhydrides,ketimines, diethylenetriamine (DETA), triethylene-tetramine (TETA),tetraethylene-pentamine (TEPA), polyethylene polyamine, polyamine epoxyadduct, phenolic hardener, non-brominated curing agent, non-aminecuratives, and combinations thereof.
 14. The method of claim 7, whereinsaid chemical functional group is selected from the group consisting ofOY, NHY, O═C—OY, P═C—NR′Y, O═C—SY, O═C—Y, —CR′1-OY, N′Y or C′Y, and Y isa functional group of a protein, a peptide, an amino acid, an enzyme, anantibody, a nucleotide, an oligonucleotide, an antigen, or an enzymesubstrate, enzyme inhibitor or the transition state analog of an enzymesubstrate or is selected from the group consisting of R′—OH, R′—NR′₂,R′SH, R′CHO, R′CN, R′X, R′N⁺(R′)₃X⁻, R′SiR′₃, R′Si(—OR′—)_(y)R′_(3−y),R′Si(—O—SiR′₂—)OR′, R′—R″, R′—N—CO, (C₂H₄O—)_(w)H, (—C₃H₆O—)_(w)H,(—C₂H₄O)_(w)—R′, (C₃H₆O)_(w)—R′, R′, and w is an integer greater thanone and less than
 200. 15. The method of claim 1, wherein said alkalimetal-selenium battery is selected from a rechargeable lithium-seleniumcell, sodium-selenium cell, potassium-selenium cell, lithiumion-selenium cell, sodium ion-selenium cell, or potassium ion-seleniumcell.
 16. The method of claim 1, wherein said electrolyte is selectedfrom the group consisting of polymer electrolyte, polymer gelelectrolyte, composite electrolyte, ionic liquid electrolyte,non-aqueous liquid electrolyte, soft matter phase electrolyte,solid-state electrolyte, and combinations thereof.
 17. The method ofclaim 1, wherein said electrolyte contains an alkali salt selected fromthe group consisting of lithium perchlorate (LiClO₄), lithiumhexafluorophosphate (LiPF₆), lithium borofluoride (LiBF₄), lithiumhexafluoroarsenide (LiAsF₆), lithium trifluoro-metasulfonate (LiCF₃SO₃),bis-trifluoromethyl sulfonylimide lithium (LiN(CF₃SO₂)₂, lithiumbis(oxalato)borate (LiBOB), lithium oxalyldifluoroborate (LiBF₂C₂O₄),lithium oxalyldifluoroborate (LiBF₂C₂O₄), lithium nitrate (LiNO₃),Li-fluoroalkyl-phosphate (LiPF3(CF₂CF₃)₃), lithiumbisperfluoroethysulfonylimide (LiBETI), an ionic liquid salt, sodiumperchlorate (NaClO₄), potassium perchlorate (KC10₄), sodiumhexafluorophosphate (NaPF₆), potassium hexafluorophosphate (KPF₆),sodium borofluoride (NaBF₄), potassium borofluoride (KBF₄), sodiumhexafluoroarsenide, potassium hexafluoroarsenide, sodiumtrifluoro-metasulfonate (NaCF₃SO₃), potassium trifluoro-metasulfonate(KCF₃SO₃), bis-trifluoromethyl sulfonylimide sodium (NaN(CF₃SO₂)₂),sodium trifluoromethanesulfonimide (NaTFSI), bis-trifluoromethylsulfonylimide potassium (KN(CF₃SO₂)₂), and combinations thereof.
 18. Themethod of claim 1, wherein said electrolyte contains a solvent selectedfrom the group consisting of ethylene carbonate (EC), dimethyl carbonate(DMC), methylethyl carbonate (MEC), diethyl carbonate (DEC), ethylpropionate, methyl propionate, propylene carbonate (PC), γ-butyrolactone(γ-BL), acetonitrile (AN), ethyl acetate (EA), propyl formate (PF),methyl formate (MF), toluene, xylene or methyl acetate (MA),fluoroethylene carbonate (FEC), vinylene carbonate (VC), allyl ethylcarbonate (AEC), 1,3-dioxolane (DOL), 1,2-dimethoxyethane (DME),tetraethylene glycol dimethylether (TEGDME), poly(ethylene glycol)dimethyl ether (PEGDME), diethylene glycol dibutyl ether (DEGDBE),2-ethoxyethyl ether (EEE), sulfone, sulfolane, room temperature ionicliquid, and combinations thereof.
 19. The method of claim 1, whereinsaid cathode active material layer contains a cathode active materialSe, metal selenide, a second element selected from Sn, Sb, Bi, S, Te, ora combination thereof and said cathode active material is in a form ofthin coating or particles having a thickness of diameter from 0.5 nm to100 nm.
 20. The method of claim 19, wherein said thin coating orparticles reside in pores or bonded to pore walls of a carbon-based,graphite-based, or graphene-based foam structure.